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>

diff --cc sound/pci/hda/hda_generic.c
Simple merge

diff --cc sound/pci/hda/hda_generic.h
Simple merge

diff --cc sound/pci/hda/patch_analog.c
Simple merge

diff --cc sound/pci/hda/patch_conexant.c
Simple merge

diff --cc sound/pci/hda/patch_via.c
Simple merge