ALSA: hda - Make add_stereo_mix_input flag tristate

... for distinguishing whether it's explicitly enabled via a user hint
or enabled by a driver as a fallback.  Now the former case corresponds
to HDA_HINT_STEREO_MIX_ENABLE while the latter to
HDA_HINT_STEREO_MIX_AUTO.

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