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 --git a/sound/pci/hda/hda_generic.c b/sound/pci/hda/hda_generic.c
index 293af6b..b680b4e 100644
--- a/sound/pci/hda/hda_generic.c
+++ b/sound/pci/hda/hda_generic.c
@@ -3218,8 +3218,7 @@
 	}
 
 	/* add stereo mix when explicitly enabled via hint */
-	if (mixer && spec->add_stereo_mix_input &&
-	    snd_hda_get_bool_hint(codec, "add_stereo_mix_input") > 0) {
+	if (mixer && spec->add_stereo_mix_input == HDA_HINT_STEREO_MIX_ENABLE) {
 		err = parse_capture_source(codec, mixer, CFG_IDX_MIX, num_adcs,
 					   "Stereo Mix", 0);
 		if (err < 0)
@@ -4544,9 +4543,8 @@
 
 	/* add stereo mix if available and not enabled yet */
 	if (!spec->auto_mic && spec->mixer_nid &&
-	    spec->add_stereo_mix_input &&
-	    spec->input_mux.num_items > 1 &&
-	    snd_hda_get_bool_hint(codec, "add_stereo_mix_input") < 0) {
+	    spec->add_stereo_mix_input == HDA_HINT_STEREO_MIX_AUTO &&
+	    spec->input_mux.num_items > 1) {
 		err = parse_capture_source(codec, spec->mixer_nid,
 					   CFG_IDX_MIX, spec->num_all_adcs,
 					   "Stereo Mix", 0);