[media] cx231xx: fix tuner compliance issues

The g_tuner call wasn't passed on to the subdevices, g_frequency didn't
check for invalid tuners and a low-level function that was expected to
return 0 or a negative error returned a positive number instead, causing
s_frequency to return bogus errors.

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