audio: ac97: add exit function

Currently the ac97 device emulation doesn't have a exit function,
hot unplug this device will leak some memory. Add a exit function to
avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 58520052.4825ed0a.27a71.6cae@mx.google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
