V4L/DVB (9653): em28xx: improve AC97 handling

AC97 devices provide several input and outputs. However, before this
patch, em28xx device weren't properly allowing the usage of ac97
possible combinations. Also, several input volumes were left untouched,
instead of making sure that the volumes were set on mute state.

This patch improves support for ac97 devices by allowing to use any
inputs, and making sure that unused inputs are set on mute state.

Yet, some work is still needed to select the AC97 output.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>