i40e: Fix Rx checksum error counter

The Rx port checksum error counter was incrementing incorrectly with
UDP encapsulated tunneled traffic.  This patch fixes the problem so that
the port_rx_csum counter will show accurate statistics.

Signed-off-by: Anjali Singhai <anjali.singhai@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>