media: atomisp: use LINUX_VERSION_CODE for driver version

The atomisp subdev driver hardcodes its version to
ATOMISP_CSS_VERSION_21. Yet, it has several tests for versions
below 21 internally, with sounds really odd.

On all other media drivers, we're just keeping version set to
LINUX_VERSION_CODE.

So, do the same here, simplifying the code a little bit.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>