V4L/DVB (10833): em28xx: enable Compro VideoMate ForYou sound

Compro VideoMate uses an external audio DSP chip, controlled via tvaudio
module (tda9874a). This patch improves em28xx infrastructure to support
an external audio processor and fixes the Compro VideoMate entry to work
with it.

Signed-off-by: Vitaly Wool <vital@embeddedalley.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>