media: staging: atomisp-gc2235: don't fill an unused var

The code with uses the dummy var is commented out. So,
coment out its definition/initialization.

Fix this warning:

  drivers/staging/media/atomisp/i2c/atomisp-gc2235.c: In function 'gc2235_get_intg_factor':
  drivers/staging/media/atomisp/i2c/atomisp-gc2235.c:249:26: warning: variable 'dummy' set but not used [-Wunused-but-set-variable]
    u16 reg_val, reg_val_h, dummy;
                            ^~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>