[media] radio-bcm2048: fix missing unlock on error in bcm2048_rds_fifo_receive()

Add the missing unlock before return from function bcm2048_rds_fifo_receive()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>