i40e/i40evf: Clean-up Rx packet checksum handling

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>