ALSA: usb/mixer - use get_iface_desc() rather than direct structure

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

sound/usb/mixer.c

@@ -2055,7 +2055,7 @@ int snd_usb_create_mixer(struct snd_usb_audio *chip, int ctrlif,
 	}
 
 	host_iface = &usb_ifnum_to_if(chip->dev, ctrlif)->altsetting[0];
-	mixer->protocol = host_iface->desc.bInterfaceProtocol;
+	mixer->protocol = get_iface_desc(host_iface)->bInterfaceProtocol;
 
 	if ((err = snd_usb_mixer_controls(mixer)) < 0 ||
 	    (err = snd_usb_mixer_status_create(mixer)) < 0)