[media] tuner-core/simple: get_rf_strength can be tuner mode specific

The get_rf_strength op in tuner-simple is valid only for the radio mode.
But due to the way get_signal in analog_demod_ops was designed it would
overwrite the signal value with a bogus value when in TV mode.
Pass a pointer to the signal value instead, and when not in radio mode
leave it alone in the tuner-simple.
This broke in commit 030755bd


(tuner-core: call has_signal for both TV and radio) in kernel 3.6. Before
that this was working correctly. That commit did the right thing, but what
wasn't realized at the time was that tuner-simple should have been updated
as well to restrict setting the signal strength to the radio mode only.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>