ALSA: usb-audio: Fix memory leak in FTU quirk

M-audio FastTrack Ultra quirk doesn't release the kzalloc'ed memory.
This patch adds the private_free callback to release it properly.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_quirks.c

@@ -885,6 +885,11 @@ static int snd_ftu_eff_switch_put(struct snd_kcontrol *kctl,
 	return changed;
 }
 
+static void kctl_private_value_free(struct snd_kcontrol *kctl)
+{
+	kfree((void *)kctl->private_value);
+}
+
 static int snd_ftu_create_effect_switch(struct usb_mixer_interface *mixer,
 	int validx, int bUnitID)
 {
@@ -919,6 +924,7 @@ static int snd_ftu_create_effect_switch(struct usb_mixer_interface *mixer,
 		return -ENOMEM;
 	}
 
+	kctl->private_free = kctl_private_value_free;
 	err = snd_ctl_add(mixer->chip->card, kctl);
 	if (err < 0)
 		return err;