iwlwifi: separate firmware version for 7260 devices

The 7260 devices aren't going to be updated for completely new
firmware versions any more (only bugfixes), and haven't been
since API version 17. Encode that in the data structures to
avoid trying to load FW images that will never exist.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>