media: 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.
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/usb/usbtv/usbtv-video.c

@@ -781,6 +781,8 @@ 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)