[media] au0828: frequency handling fixes

- define an initial frequency
- return an error if g_frequency is called for an invalid tuner index
- get the clamped frequency value after setting it: i.e. the tuner driver
  may clamp the given frequency to a valid frequency range and ctrl_freq
  should get that actual clamped frequency.
- remove obsolete tuner type checks (done by the core).

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