[media] tvaudio: rename getmode and setmode

This is basically s/getmode/getrxsubchans/ and s/setmode/setaudmode/
with some whitespace adjustment in affected lines to please the eye.
The rename is done to point out their relation to the rxsubchans and
audmode fields of struct v4l2_tuner.

I also corrected a commented out call to v4l_dbg in one of the lines.

Signed-off-by: Daniel Glöckner <daniel-gl@gmx.net>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>