[media] dvb_frontend: get rid of set_state ops & related data

The get_state()/set_state and the corresponding data types
(struct tuner_state and enum tuner_param) are old DVB interfaces
that came from the DVBv3 time.

Nowadays, set_params() provide a better way to set the tuner
and demod parameters. So, no need to keep those legacy stuff,
as all drivers that were using it got converted.

With this patch, all kABI elements at dvb_frontend.h are now
documented.

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