ALSA: scarlett2: Allow for interfaces without per-channel volume

Currently-supported interfaces with a mixer have per-channel volume
controls, but this changes in Gen 4. Add a check so that the Playback
Volume and associated controls don't get created unless the
SCARLETT2_CONFIG_LINE_OUT_VOLUME config item is present.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/30f68cb311e27f2cc1351cb846218f7248a90263.1703444932.git.g@b4.vuSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_scarlett2.c

@@ -3726,6 +3726,13 @@ static int scarlett2_add_line_out_ctls(struct usb_mixer_interface *mixer)
 			return err;
 	}
 
+	/* Remaining controls are only applicable if the device
+	 * has per-channel line-out volume controls.
+	 */
+	if (!scarlett2_has_config_item(private,
+				       SCARLETT2_CONFIG_LINE_OUT_VOLUME))
+		return 0;
+
 	/* Add volume controls */
 	for (i = 0; i < private->num_line_out; i++) {
 		int index = line_out_remap(private, i);
@@ -4569,7 +4576,6 @@ static int scarlett2_read_configs(struct usb_mixer_interface *mixer)
 {
 	struct scarlett2_data *private = mixer->private_data;
 	int err, i;
-	s16 sw_vol[SCARLETT2_ANALOGUE_MAX];
 
 	if (scarlett2_has_config_item(private, SCARLETT2_CONFIG_MSD_SWITCH)) {
 		err = scarlett2_usb_get_config(
@@ -4608,6 +4614,10 @@ static int scarlett2_read_configs(struct usb_mixer_interface *mixer)
 	if (err < 0)
 		return err;
 
+	if (scarlett2_has_config_item(private,
+				      SCARLETT2_CONFIG_LINE_OUT_VOLUME)) {
+		s16 sw_vol[SCARLETT2_ANALOGUE_MAX];
+
 		/* read SW line out volume */
 		err = scarlett2_usb_get_config(
 			mixer, SCARLETT2_CONFIG_LINE_OUT_VOLUME,
@@ -4636,7 +4646,8 @@ static int scarlett2_read_configs(struct usb_mixer_interface *mixer)
 					      SCARLETT2_CONFIG_SW_HW_SWITCH)) {
 			err = scarlett2_usb_get_config(
 				mixer, SCARLETT2_CONFIG_SW_HW_SWITCH,
-			private->num_line_out, &private->vol_sw_hw_switch);
+				private->num_line_out,
+				&private->vol_sw_hw_switch);
 			if (err < 0)
 				return err;
 
@@ -4644,6 +4655,7 @@ static int scarlett2_read_configs(struct usb_mixer_interface *mixer)
 				private->vol_sw_hw_switch[i] =
 					!!private->vol_sw_hw_switch[i];
 		}
+	}
 
 	err = scarlett2_update_volumes(mixer);
 	if (err < 0)