[media] radio-miropcm20: Fix audmode/tuner/frequency handling

- instead of a mute module option, use audmode as per the spec.
- clamp the frequency before setting it.

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