[media] em28xx-dvb: create RF connector on DVB-only mode

When in analog mode, the RF connector will be created by
em28xx-video. However, when the device is in digital mode only,
the RF connector is not shown. In this case, let the DVB
core to create it for us.

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