media: radio: sprd: fix only requesting to get snr when used

media: radio: sprd: fix only requesting to get snr when used

When not used, requesting to get snr causes following kernel panic
because driver is not initialized. Fix only requesting to get snr
when used.

   Internal error: Oops: 805 [#1] PREEMPT SMP ARM
   ...
   [<c05a9fd8>] (skb_queue_tail+0x34/0x58) from [<c047e7e4>] (__fm_send_cmd.isra.0+0xcc/0xf8)
   [<c047e7e4>] (__fm_send_cmd.isra.0+0xcc/0xf8) from [<c047ea5c>] (fmc_send_cmd+0x6c/0x210)
   [<c047ea5c>] (fmc_send_cmd+0x6c/0x210) from [<c0482764>] (fm_rx_get_snr+0x4c/0x84)
   [<c0482764>] (fm_rx_get_snr+0x4c/0x84) from [<c0480ad4>] (show_fmrx_curr_snr+0x34/0x6c)
   [<c0480ad4>] (show_fmrx_curr_snr+0x34/0x6c) from [<c0308264>] (dev_attr_show+0x30/0x54)
   [<c0308264>] (dev_attr_show+0x30/0x54) from [<c0172498>] (sysfs_read_file+0xc0/0x170)
   [<c0172498>] (sysfs_read_file+0xc0/0x170) from [<c01160b8>] (vfs_read+0xbc/0x138)
   [<c01160b8>] (vfs_read+0xbc/0x138) from [<c01166a8>] (SyS_read+0x50/0x78)
   [<c01166a8>] (SyS_read+0x50/0x78) from [<c000fa4c>] (__sys_trace_return+0x0/0x14)

Change-Id: I6ada090c40925212ef96107d56021612c5cf46fe
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>