media: staging/intel-ipu3: Constify static struct v4l2_subdev_internal_ops

The only usage of imgu_subdev_internal_ops is to assign its address to
the internal_ops field in the v4l2_subdev struct, which is a pointer to
const v4l2_subdev_internal_ops. Make it const to allow the compiler to
put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>