ALSA: scarlett2: Add missing error check to scarlett2_config_save()

scarlett2_config_save() was ignoring the return value from
scarlett2_usb(). As this function is not called from user-space we
can't return the error, so call usb_audio_err() instead.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Fixes: 9e4d5c1b ("ALSA: usb-audio: Scarlett Gen 2 mixer interface")
Link: https://lore.kernel.org/r/bf0a15332d852d7825fa6da87d2a0d9c0b702053.1703001053.git.g@b4.vuSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_scarlett2.c

@@ -1524,9 +1524,11 @@ static void scarlett2_config_save(struct usb_mixer_interface *mixer)
 {
 	__le32 req = cpu_to_le32(SCARLETT2_USB_CONFIG_SAVE);
 
-	scarlett2_usb(mixer, SCARLETT2_USB_DATA_CMD,
-		      &req, sizeof(u32),
-		      NULL, 0);
+	int err = scarlett2_usb(mixer, SCARLETT2_USB_DATA_CMD,
+				&req, sizeof(u32),
+				NULL, 0);
+	if (err < 0)
+		usb_audio_err(mixer->chip, "config save failed: %d\n", err);
 }
 
 /* Delayed work to save config */