iwlagn: don't check ucode subtype

The ucode subtypes keep changing, and there's no
particular reason to be checking them (other than
a paranoid sanity check). Since the numbers are
also in conflict between different ucode images
now, simply don't check them any more and rely on
the images being built correctly.

Also, to indicate that, rename the constants and
the enum, moving it to a different file.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>