V4L/DVB (9670): em28xx: allow specifying audio output

Some boards use different AC97 setups for output. This patch adds the
capability of specifying the output to be used. Currently, only one
output is selected, but the better is to allow user to select it via a
mixer, on alsa driver.

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