media: xen-front: Don't mark buffer as queued in prepare

Marking the buffer as queued in prepare causes it to sometimes get
processed before the queue function is called and the v4l is ready to
handle it, that causes warnings.

The same flag is already being set in the queue function. So skip the
setting in prepare.

Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>

drivers/media/xen/xen_camera_front_v4l2.c

@@ -476,9 +476,6 @@ static int buffer_prepare(struct vb2_buffer *vb)
 		return ret;
 	}
 
-	mutex_lock(&v4l2_info->bufs_lock);
-	xen_buf->is_queued = true;
-	mutex_unlock(&v4l2_info->bufs_lock);
 	return 0;
 }