[media] lirc_dev: avoid double mutex unlock

We can only unlock if mutex_lock() succeeds.

Fixes the following warning:
	drivers/media/rc/lirc_dev.c:535 lirc_dev_fop_close() error: double unlock 'mutex:&lirc_dev_lock'

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>