[media] vb2: Return 0 when streamon and streamoff are already on/off

According to the doc:
If VIDIOC_STREAMON is called when streaming is already in progress,
or if VIDIOC_STREAMOFF is called when streaming is already stopped,
then the ioctl does nothing and 0 is returned.
The current implementation was returning -EINVAL instead.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -1697,8 +1697,8 @@ int vb2_streamon(struct vb2_queue *q, enum v4l2_buf_type type)
 	}
 
 	if (q->streaming) {
-		dprintk(1, "streamon: already streaming\n");
-		return -EBUSY;
+		dprintk(3, "streamon successful: already streaming\n");
+		return 0;
 	}
 
 	/*
@@ -1754,8 +1754,8 @@ int vb2_streamoff(struct vb2_queue *q, enum v4l2_buf_type type)
 	}
 
 	if (!q->streaming) {
-		dprintk(1, "streamoff: not streaming\n");
-		return -EINVAL;
+		dprintk(3, "streamoff successful: not streaming\n");
+		return 0;
 	}
 
 	/*