[media] au0828: fix audio input handling

- V4L2_CAP_AUDIO was set, but enumaudio was not implemented.
- audioset was never filled by enum_input
- ctrl_ainput was never updated when switching the video input
- g_audio was broken due to faulty logic: g_audio should set the
  index, it doesn't receive it from the user.

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>