[media] rc-core: cleanup rc_register_device

The device core infrastructure is based on the presumption that
once a driver calls device_add(), it must be ready to accept
userspace interaction.

This requires splitting rc_setup_rx_device() into two functions
and reorganizing rc_register_device() so that as much work
as possible is performed before calling device_add().

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>