ALSA: hda - Create capture source ctls when stereo mix input is added

When the stereo mix input is explicitly enabled via a user hint, the
driver should create always a capture source enum ctl and disable the
auto-mic switch.  Otherwise the behavior gets confused.  For doing it,
this patch just sets spec->suppress_auto_mic flag appropriately.

Signed-off-by: Takashi Iwai <tiwai@suse.de>