[ALSA] hda-codec - Don't build boost controls for digital mics

The ALC auto-probe creates mic boost controls automatically for the
probed pins, but it assumes that they are analog mics.  The digital
mics have no boost controls and must be skipped.

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