media: atmel-isc: get rid of an unused var

drivers/media/platform/atmel/atmel-isc.c: In function 'isc_async_complete':
drivers/media/platform/atmel/atmel-isc.c:1900:28: warning: variable 'sd_entity' set but not used [-Wunused-but-set-variable]
  struct isc_subdev_entity *sd_entity;
                            ^~~~~~~~~

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