ALSA: snd-usb-6fire: add mute control for analog outputs

Add a mute control for every analog output channel.

Signed-off-by: Torsten Schenk <torsten.schenk@zoho.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/6fire/control.h b/sound/usb/6fire/control.h
index ce02411..9f9eb64 100644
--- a/sound/usb/6fire/control.h
+++ b/sound/usb/6fire/control.h
@@ -45,6 +45,7 @@
 	bool usb_streaming;
 	u8 output_vol[6];
 	u8 ovol_updated;
+	u8 output_mute;
 };
 
 int __devinit usb6fire_control_init(struct sfire_chip *chip);