media: adv748x: get rid of unused var

drivers/media/i2c/adv748x/adv748x-csi2.c: In function 'adv748x_csi2_init_controls':
drivers/media/i2c/adv748x/adv748x-csi2.c:251:20: warning: variable 'ctrl' set but not used [-Wunused-but-set-variable]
  struct v4l2_ctrl *ctrl;
                    ^~~~

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