[media] rc-main: fix missing unlock if no devno left

While playing with make coccicheck I noticed this message:
drivers/media/rc/rc-main.c:1245:3-9: preceding lock on line 1238

It was introduced by commit 587d1b06 ([media] rc-core: reuse device
numbers) which returns -ENOMEM after a mutex_lock without first
unlocking it when there are no more device numbers left. The added code
doesn't depend on the device lock, so move it before the lock is taken.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>