V4L/DVB (3864): Convert dvb_dummy_fe to refactored tuner code

Make it call the tuner calls - in case someone wants to use it to develop a
tuner without a demod.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>