media: atomisp: change the detection of ISP2401 at runtime

Instead of having a static var to detect it, let's use the
already-existing arch-specific bytes, as this is how other
parts of the code also checks when it needs to do something
different, depending on an specific chipset version.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>