i40e: add little endian conversion for checksum

The checksum is not correct on big endian machines so add code to swap it
correctly.

Change-ID: Ic92b886d172a2cbe49f5d7eee1bc78e447023c7b
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Paul M Stillwell Jr <paul.m.stillwell.jr@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>