media: rkisp1: isp: Add container_of wrapper to cast subdev to rkisp1_isp

Replace manual container_of() calls with a static inline wrapper to
increase readability.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>