[media] usbvision: frequency fixes

- setup initial radio and tv frequencies.
- set/get the correct frequency (radio vs tv).
- disable tuner/freq ioctls if there is no tuner.
- fix some tuner index checks.

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