iwlwifi: bump required firmware API version for 3160/7260

A new firmware is coming out soon with new APIs.
To make sure that this new firmware won't be loaded on old
driver that don't support it, it's API version has been
updated to 8. In order to be able to load it, bump the API
version to 8.
API version 7 is still supported and will be for another
year or so.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>