[ALSA] hda-codec - Fix invalid access to non-existing dmux on STAC

The digital mux on STAC codecs doesn't always exist although the
driver builds dmux enum mixer elements unconditionally.
Now the driver creates 'digital input source' mixer elements only
when dmux is available.
Also, the patch adds the missing dmux definition for STAC925x.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>