[media] dvb: simplify get_tune_settings() struct

In the past, dvb_frontent_parameters were passed inside the
struct where get_tuner_settings should store their result.

This is not needed anymore, as all parameters needed are stored
already at the fe property cache. So, use it, where needed.

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