media: rkisp1: Mark subdev state pointers as const

Several subdev state pointers are used to access the state read-only.
Mark them as const.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20240508214045.24716-4-laurent.pinchart+renesas@ideasonboard.com