hw/audio: convert exit callback in HDACodecDeviceClass to void

The exit callback always return 0, convert it to void

Signed-off-by: Zihan Yang <tgnyang@gmail.com>
Message-id: 1493211188-24086-5-git-send-email-tgnyang@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>