[ALSA] hda: Add dmux to STAC 9228

Added a dmux to the STAC9228 cards with DMIC support. And added a
STAC_DIGITAL_INPUT_SOURCE macro for repeating mixer code.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>