V4L/DVB (8999): S2API: Reduce demod driver complexity by using a cache sync

For demod drivers, we want a single tuning function (set_frontend) to have
access to the properly constructed dvb_frontend_parameters structure, or
the cache values - regardless of whether the legacy or new API is being used.

This cuts down on redundant code in the demod drivers and ensures the
tuning parameters are consistent regardless of the API entry type.

Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>