i40e: Changed maximum supported FW API version to 1.8

A new FW has been released, which uses API version 1.8.

Signed-off-by: Adam Ludkiewicz <adam.ludkiewicz@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>