i40e/i40evf: Clean-up process_skb_fields

This is a minor clean-up to make the i40e/i40evf process_skb_fields
function look a little more like what we have in igb.  The Rx checksum
function called out a need for skb->protocol but I can't see where it
actually needs it.  I am assuming this is something that was likely
refactored out some time ago as the Rx checksum code has gone through a few
rewrites.

Change-ID: I0b4668a34d90b61b66ded7c7c26e19a3e2d06251
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>