media: lirc: improve locking

Once rc_unregister_device() has been called, no driver function
should be called.

This prevents some nasty race conditions with an ioctl calls
driver functions when the driver specific data has been freed.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>