[media] atmel-isi: get rid of a warning

drivers/media/platform/soc_camera/atmel-isi.c: In function 'start_streaming':
drivers/media/platform/soc_camera/atmel-isi.c:387:6: warning: variable 'sr' set but not used [-Wunused-but-set-variable]
  u32 sr = 0;
      ^

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>