[media] tuner-core: don't set has_signal/get_afc if not supported

If the tuner frontend does not support get_rf_strength, then don't set
the has_signal callback. Ditto for get_afc.
Both callbacks overwrite the signal and afc fields of struct v4l2_tuner
but that should only happen if the tuner can actually detect this. If
it can't, then it should leave those fields alone so other subdevices
can try and detect the signal/afc.
This fixes the bug where the au8522 detected a signal and then tuner-core
overwrote it with 0 since the xc5000 tuner does not support get_rf_strength.

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