V4L/DVB (3845): DVB core changes for PLL refactoring

Add tuner_ops structure.
Add calls into dvb_frontend to support the new tuner architecture.

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