i40e: add GRE tunnel type to csum encoding

Make sure the Tx checksum encoder knows about GRE protocol and sets the
descriptor flag appropriately.

Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>