iwlwifi: Update iwlwifi version stamp to 1.2.26

We update the version stamp at this time to distinguish the in-kernel
driver from the out of tree driver. The out of tree driver currently
has version number 1.2.25, but the latest driver code can only be
found in this in-kernel driver.

Having a later version number will reduce confusion between the two
versions as we transition from out of tree to in-kernel.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>