BCM2835-V4L2: Return buffers to videobuf2 on shutdown

#817
Fixes the kernel warning from videobuf2 as buffers
are now returned as they are being flushed on
stop_streaming.

drivers/media/platform/bcm2835/bcm2835-camera.c

@@ -404,6 +404,7 @@ static void buffer_cb(struct vchiq_mmal_instance *instance,
  }
  } else {
  /* signal frame completion */
+ vb2_buffer_done(&buf->vb, VB2_BUF_STATE_ERROR);
  complete(&dev->capture.frame_cmplt);
  }
  }