i40e/i40evf: Clean-up Rx packet checksum handling

i40e/i40evf: Clean-up Rx packet checksum handling

BugLink: http://bugs.launchpad.net/bugs/1547674
This is mostly a minor clean-up for the Rx checksum path in order to avoid
some of the unnecessary conditional checks that were being applied.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from net-next commit fad57330b6d0710fdf39dc1c2b28ccebb97ae8a1)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>