[ALSA] fix usbmixer double kfree

snd_ctl_add() kfree's the kcontrol already if we fail there,
so this driver is currently doing a double kfree.

Coverity bug #959
Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

sound/usb/usbmixer.c

@@ -434,7 +434,6 @@ static int add_control_to_empty(struct mixer_build *state, struct snd_kcontrol *
 		kctl->id.index++;
 	if ((err = snd_ctl_add(state->chip->card, kctl)) < 0) {
 		snd_printd(KERN_ERR "cannot add control (err = %d)\n", err);
-		snd_ctl_free_one(kctl);
 		return err;
 	}
 	cval->elem_id = &kctl->id;