[media] rcar-vin: get rid of an unused var

drivers/media/platform/rcar-vin/rcar-core.c: In function 'rvin_graph_notify_complete':
drivers/media/platform/rcar-vin/rcar-core.c:65:22: warning: variable 'sd' set but not used [-Wunused-but-set-variable]
  struct v4l2_subdev *sd;
                      ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>