V4L/DVB (10228): em28xx: fix audio output PCM IN selection

Some em28xx devices use the PCM IN AC 97 PIN for digital audio. However,
currently, the PCM IN selection is not set by the driver. This patch allows
specifying the PCM IN expected output, via board description table.

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