[media] em28xx: make 'em28xx_ctrl_ops' static

sparse warnings: (new ones prefixed by >>)

>> drivers/media/usb/em28xx/em28xx-video.c:1151:28: sparse: symbol 'em28xx_ctrl_ops' was not declared. Should it be static?
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NMauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/usb/em28xx/em28xx-video.c

@@ -1147,7 +1147,7 @@ static int em28xx_s_ctrl(struct v4l2_ctrl *ctrl)
 	return (ret < 0) ? ret : 0;
 }
 
-const struct v4l2_ctrl_ops em28xx_ctrl_ops = {
+static const struct v4l2_ctrl_ops em28xx_ctrl_ops = {
 	.s_ctrl = em28xx_s_ctrl,
 };