ALSA: hda: SPDIF stream muting support

Added support for muting S/DPIF outs using the 'Digital Playback Source' muxs on
supporting codecs.

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