[media] tuner-core.c: don't change type field in g_tuner or g_frequency

The tuner core should not silently change the type field in g_tuner and
g_frequency. If the tuner is in a different mode than the one that was
requested, then just fill in what you can and don't attempt to read afc,
signal or rxsubchans values.

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