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

commit 1a290581ded60e87276741f8ca97b161d2b226fc upstream.

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

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>