V4L/DVB: dvb-usb: prepare drivers for using rc-core

This is a big patch, yet trivial. It just move the RC properties
to a separate struct, in order to prepare the dvb-usb drivers to
use rc-core. There's no change on the behavior of the drivers.

With this change, it is possible to have both legacy and rc-core
based code inside the dvb-usb-remote, allowing a gradual migration
to rc-core, driver per driver.

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