[ALSA] hda: support for S/PDIF out on ASUS M2V

Added support for S/PDIF digital output on ASUS M2V motheboard - added
new model '3stack-660-digout' and ALC660VD_3ST_DIG

Signed-off-by: Mike Crash <mike@mikecrash.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>