[media] mx2_camera: get rid of a warning

drivers/media/platform/soc_camera/mx2_camera.c: In function 'mx27_camera_emma_prp_reset':
drivers/media/platform/soc_camera/mx2_camera.c:812:6: warning: variable 'cntl' set but not used [-Wunused-but-set-variable]
  u32 cntl;
      ^

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