i40evf: Fix Outer UDP RX checksum code

Inner protocol being UDP should not stop us from verifying Outer UDP
checksum correctness.

If the Outer protocol is not UDP (NVGRE) we should not be doing a UDP
checksum check. If the packet has zero checksum, skip checksum check.

Change-ID: Ie7f153feb276a59f66a54a0938901b2c0a8100fa
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>