[media] em28xx: only initialize extensions on the main interface

[media] em28xx: only initialize extensions on the main interface

For devices with a separated audio-only interface (em2860), call
em28xx_init_extension() only once.

That fixes a bug with Kworld 305U (eb1a:e305):

    [  658.730715] em2860 #0: V4L2 video device registered as video1
    [  658.730728] em2860 #0: V4L2 VBI device registered as vbi0
    [  658.736907] em2860 #0: Remote control support is not available for this card.
    [  658.736965] em2860 #1: Remote control support is not available for this card.
    [  658.737230] ------------[ cut here ]------------
    [  658.737246] WARNING: CPU: 2 PID: 60 at lib/list_debug.c:36 __list_add+0x8a/0xc0()
    [  658.737256] list_add double add: new=ffff8800a9a40410, prev=ffff8800a9a40410, next=ffffffffa08720d0.
    [  658.737266] Modules linked in: tuner_xc2028 netconsole rc_hauppauge em28xx_rc rc_core tuner_simple tuner_types tda9887 tda8290 tuner tvp5150 msp3400 em28xx_v4l em28xx tveeprom
 v4l2_common fuse ccm nf_conntrack_netbios_ns nf_conntrack_broadcast ipt_MASQUERADE ip6t_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6tabl
e_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4
nf_nat nf_conntrack iptable_mangle iptable_security bnep iptable_raw vfat fat arc4 iwldvm mac80211 x86_pkg_temp_thermal coretemp kvm_intel nfsd iwlwifi snd_hda_codec_hdmi kvm snd_hda
_codec_realtek snd_hda_intel snd_hda_codec auth_rpcgss nfs_acl cfg80211 lockd snd_hwdep snd_seq btusb sunrpc crc32_pclmul bluetooth crc32c_intel snd_seq_device snd_pcm uvcvideo r8169
 ghash_clmulni_intel videobuf2_vmalloc videobuf2_memops videobuf2_core snd_page_alloc snd_timer snd videodev mei_me iTCO_wdt mii shpchp joydev mei media iTCO_vendor_support lpc_ich m
icrocode soundcore rfkill serio_raw i2c_i801 mfd_core nouveau i915 ttm i2c_algo_bit drm_kms_helper drm i2c_core mxm_wmi wmi video
    [  658.738601] CPU: 2 PID: 60 Comm: kworker/2:1 Not tainted 3.13.0-rc1+ #18
    [  658.738611] Hardware name: SAMSUNG ELECTRONICS CO., LTD. 550P5C/550P7C/SAMSUNG_NP1234567890, BIOS P04ABI.013.130220.dg 02/20/2013
    [  658.738624] Workqueue: events request_module_async [em28xx]
    [  658.738646]  0000000000000009 ffff8802209dfc68 ffffffff816a3c96 ffff8802209dfcb0
    [  658.738700]  ffff8802209dfca0 ffffffff8106aaad ffff8800a9a40410 ffffffffa08720d0
    [  658.738754]  ffff8800a9a40410 0000000000000000 0000000000000080 ffff8802209dfd00
    [  658.738814] Call Trace:
    [  658.738836]  [<ffffffff816a3c96>] dump_stack+0x45/0x56
    [  658.738851]  [<ffffffff8106aaad>] warn_slowpath_common+0x7d/0xa0
    [  658.738864]  [<ffffffff8106ab1c>] warn_slowpath_fmt+0x4c/0x50
    [  658.738880]  [<ffffffffa0868a7d>] ? em28xx_init_extension+0x1d/0x80 [em28xx]
    [  658.738898]  [<ffffffff81343b8a>] __list_add+0x8a/0xc0
    [  658.738911]  [<ffffffffa0868a98>] em28xx_init_extension+0x38/0x80 [em28xx]
    [  658.738927]  [<ffffffffa086a059>] request_module_async+0x19/0x110 [em28xx]
    [  658.738942]  [<ffffffff810873b5>] process_one_work+0x1f5/0x510
    [  658.738954]  [<ffffffff81087353>] ? process_one_work+0x193/0x510
    [  658.738967]  [<ffffffff810880bb>] worker_thread+0x11b/0x3a0
    [  658.738979]  [<ffffffff81087fa0>] ? manage_workers.isra.24+0x2b0/0x2b0
    [  658.738992]  [<ffffffff8108ea2f>] kthread+0xff/0x120
    [  658.739005]  [<ffffffff8108e930>] ? kthread_create_on_node+0x250/0x250
    [  658.739017]  [<ffffffff816b517c>] ret_from_fork+0x7c/0xb0
    [  658.739029]  [<ffffffff8108e930>] ? kthread_create_on_node+0x250/0x250
    [  658.739040] ---[ end trace c1acd24b354108de ]---
    [  658.739051] em2860 #1: Remote control support is not available for this card.
    [  658.742407] em28xx-audio.c: probing for em28xx Audio Vendor Class
    [  658.742429] em28xx-audio.c: Copyright (C) 2006 Markus Rechberger
    [  658.742440] em28xx-audio.c: Copyright (C) 2007-2011 Mauro Carvalho Chehab
    [  658.744798] em28xx-audio.c: probing for em28xx Audio Vendor Class
    [  658.744823] em28xx-audio.c: Copyright (C) 2006 Markus Rechberger
    [  658.744836] em28xx-audio.c: Copyright (C) 2007-2011 Mauro Carvalho Chehab
    [  658.746849] em28xx-audio.c: probing for em28xx Audio Vendor Class
    [  658.746863] em28xx-audio.c: Copyright (C) 2006 Markus Rechberger
    [  658.746874] em28xx-audio.c: Copyright (C) 2007-2011 Mauro Carvalho Chehab
    ...

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>