usbtv: Implement wait_prepare and wait_finish

This driver is currently specifying a vb2_queue lock, which means it straightforward to implement wait_prepare and wait_finish.

Having these callbacks releases the queue lock while blocking, which improves latency by allowing for example streamoff or qbuf operations while waiting in dqbuf.

---
Compile tested only.

 drivers/media/usb/usbtv/usbtv-video.c | 2 ++
 1 file changed, 2 insertions(+)

index 3668a04359e8..0c0d0ef71573 100644
@@ -698,6 +698,8 @@ static const struct vb2_ops usbtv_vb2_ops = {
 	.buf_queue = usbtv_buf_queue,
 	.start_streaming = usbtv_start_streaming,
 	.stop_streaming = usbtv_stop_streaming,
+	.wait_prepare = vb2_ops_wait_prepare,
+	.wait_finish = vb2_ops_wait_finish,
 };
 
 static int usbtv_s_ctrl(struct v4l2_ctrl *ctrl)

2.16.3