[media] tvaudio: obey V4L2 tuner audio matrix

V4L2 specifies the audio mode to use for combinations of possible
(rxsubchans) and requested (audmode) audio modes. Up to now tvaudio
has made these decisions automatically based on the possible audio
modes from setting of the frequency until VIDIOC_S_TUNER was called.
It then forced the hardware to use the mode requested by the user.
With this patch it continues to adjust the audio mode while taking
the requested mode into account.

Signed-off-by: Daniel Glöckner <daniel-gl@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>