llc: remove noisy WARN from llc_mac_hdr_init

llc: remove noisy WARN from llc_mac_hdr_init

Sending malformed llc packets triggers this spew, which seems excessive.

WARNING: CPU: 1 PID: 6917 at net/llc/llc_output.c:46 llc_mac_hdr_init+0x85/0x90 [llc]()
device type not supported: 0
CPU: 1 PID: 6917 Comm: trinity-c1 Not tainted 3.13.0+ #95
 0000000000000009 00000000007e257d ffff88009232fbe8 ffffffffac737325
 ffff88009232fc30 ffff88009232fc20 ffffffffac06d28d ffff88020e07f180
 ffff88009232fec0 00000000000000c8 0000000000000000 ffff88009232fe70
Call Trace:
 [<ffffffffac737325>] dump_stack+0x4e/0x7a
 [<ffffffffac06d28d>] warn_slowpath_common+0x7d/0xa0
 [<ffffffffac06d30c>] warn_slowpath_fmt+0x5c/0x80
 [<ffffffffc01736d5>] llc_mac_hdr_init+0x85/0x90 [llc]
 [<ffffffffc0173759>] llc_build_and_send_ui_pkt+0x79/0x90 [llc]
 [<ffffffffc057cdba>] llc_ui_sendmsg+0x23a/0x400 [llc2]
 [<ffffffffac605d8c>] sock_sendmsg+0x9c/0xe0
 [<ffffffffac185a37>] ? might_fault+0x47/0x50
 [<ffffffffac606321>] SYSC_sendto+0x121/0x1c0
 [<ffffffffac011847>] ? syscall_trace_enter+0x207/0x270
 [<ffffffffac6071ce>] SyS_sendto+0xe/0x10
 [<ffffffffac74aaa4>] tracesys+0xdd/0xe2

Until 2009, this was a printk, when it was changed in
bf9ae5386bc: "llc: use dev_hard_header".

Let userland figure out what -EINVAL means by itself.

Signed-off-by: Dave Jones <davej@fedoraproject.org>
Signed-off-by: David S. Miller <davem@davemloft.net>