[media] radio-si4713: fix g/s_frequency

- check for invalid modulators.
- clamp frequency to valid range.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Tested-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>