[media] ts2020: call get_rf_strength from frontend

Restore ds3000.c read_signal_strength.
Call tuner get_rf_strength from frontend read_signal_strength.
We are able to do a NULL check and doesn't limit the tuner
attach to the frontend attach area.
At the moment the lmedm04 tuner attach is stuck in frontend
attach area.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>