[media] tda9840: fix setting of the audio mode

You have to first detect the current rxsubchans (mono, stereo or bilingual),
and depending on that you can set the audio mode. It does not automatically
switch when the audio channels change.

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