iwlagn: prepare for new firmware file format

Currently the first four bytes in a firmware file
indicate the major, minor and api versions as well
as the serial number. These combined can never be
zero, so we can use that special case for a new,
future, file format.

This patch simply shuffles the code and prepares
for that new format.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>