V4L/DVB: dvb-usb: add support for rc-core mode

Allows dvb-usb drivers to use rc-core, instead of the legacy
implementation.

No driver were ported yet to rc-core, so, some small adjustments
may be needed, when starting to migrate the drivers.

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