V4L/DVB (6136): dvb_frontend: add get_rf_strength function pointer to dvb_tuner_ops

Add get_rf_strength function pointer to dvb_tuner_ops, so that rf signal
strength can be read directly from the tuner driver by the dvb demodulator
driver and / or the analog tuning system.

This is an internal api addition -- userspace is not affected.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Acked-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>