media: imx258: get rid of an unused var

drivers/media/i2c/imx258.c: In function 'imx258_init_controls':
drivers/media/i2c/imx258.c:1117:6: warning: variable 'exposure_max' set but not used [-Wunused-but-set-variable]
  s64 exposure_max;
      ^~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>