V4L/DVB (3849): Convert mt312 to refactored tuner code

Convert to tuner_ops calls.
Remove pll function pointers from structure.
Remove unneeded tuner calls.
Add i2c gate control function.

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