[media] media: lirc_dev: use an IDA instead of an array to keep track of registered devices

Using the kernel-provided IDA simplifies the code and makes it possible
to remove the lirc_dev_lock mutex.

Signed-off-by: David Härdeman <david@hardeman.nu>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>