media: atomisp: Remove atomisp_subdev_register_video_nodes() helper

Now that there is only 1 /dev/video# node left there is no need to
do this in a helper. Just make atomisp_register_device_nodes()
call video_register_device() directly.

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>