media: sta2x11: add a missing parameter description

Fixes this warning:
	drivers/media/pci/sta2x11/sta2x11_vip.c:156: warning: Function parameter or member 'v4l_lock' not described in 'sta2x11_vip'

Fixes: cd63c0288fd7 ("media: sta2x11: Add video_device and vb2_queue locks")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/pci/sta2x11/sta2x11_vip.c

@@ -116,6 +116,7 @@ static inline struct vip_buffer *to_vip_buffer(struct vb2_v4l2_buffer *vb2)
  * @sequence: sequence number of acquired buffer
  * @active: current active buffer
  * @lock: used in videobuf2 callback
+ * @v4l_lock: serialize its video4linux ioctls
  * @tcount: Number of top frames
  * @bcount: Number of bottom frames
  * @overflow: Number of FIFO overflows