ALSA: sound/usb: fix UAC1 regression

Commit 23caaf19b ("ALSA: usb-mixer: Add support for Audio Class v2.0")
broke support for Class1 devices due to two faulty changes. This patch
fixes it.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Reported-and-Tested-by: The Source <thesourcehim@gmail.com>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.c

@@ -656,7 +656,7 @@ static int check_input_term(struct mixer_build *state, int id, struct usb_audio_
 		case UAC_FEATURE_UNIT: {
 			/* the header is the same for v1 and v2 */
 			struct uac_feature_unit_descriptor *d = p1;
-			id = d->bUnitID;
+			id = d->bSourceID;
 			break; /* continue to parse */
 		}
 		case UAC_MIXER_UNIT: {