ALSA: usb-audio: Add private_data pointer to usb_mixer_elem_info

Add a private_data pointer to usb_mixer_elem_info to allow other mixer
implementations to extend the structure as necessary.
Signed-off-by: NChris J Arges <chris.j.arges@canonical.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/usb/mixer.h

@@ -53,6 +53,7 @@ struct usb_mixer_elem_info {
 	int cached;
 	int cache_val[MAX_CHANNELS];
 	u8 initialized;
+	void *private_data;
 };
 
 int snd_usb_create_mixer(struct snd_usb_audio *chip, int ctrlif,