ALSA: usb-audio: ignore broken processing/extension unit

Some devices have broken extension unit where getting current value
doesn't work. Attempt that once when creating mixer control for it. If
it fails, just ignore it, so that it won't cripple the device entirely
(and/or make the error floods).

Signed-off-by: Tom Yan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

sound/usb/mixer.c

@@ -2371,7 +2371,7 @@ static int build_audio_procunit(struct mixer_build *state, int unitid,
 	int num_ins;
 	struct usb_mixer_elem_info *cval;
 	struct snd_kcontrol *kctl;
-	int i, err, nameid, type, len;
+	int i, err, nameid, type, len, val;
 	const struct procunit_info *info;
 	const struct procunit_value_info *valinfo;
 	const struct usbmix_name_map *map;
@@ -2474,6 +2474,12 @@ static int build_audio_procunit(struct mixer_build *state, int unitid,
 			break;
 		}
 
+		err = get_cur_ctl_value(cval, cval->control << 8, &val);
+		if (err < 0) {
+			usb_mixer_elem_info_free(cval);
+			return -EINVAL;
+		}
+
 		kctl = snd_ctl_new1(&mixer_procunit_ctl, cval);
 		if (!kctl) {
 			usb_mixer_elem_info_free(cval);