media: atomisp: Set asd.subdev.devnode once from isp_subdev_init_entities()

Now that we have only one /dev/video# node we can set asd.subdev.devnode
once from isp_subdev_init_entities(), replacing the hack to set it the
last opened/closed /dev/video# node from atomisp_open() /
atomisp_release().

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>