[media] cx231xx: clean up radio support

Radio should not use video or audio inputs.
In addition, fix a bug in radio_g_tuner where s_tuner was called in the tuner
subdev instead of g_tuner.

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