[media] lirc_dev: merge lirc_register_driver() and lirc_allocate_driver()

Merging the two means that lirc_allocate_buffer() is called before
device_add() and cdev_add() which makes more sense. This also
simplifies the locking slightly because lirc_allocate_buffer() will
always be called with lirc_dev_lock held.

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