ixgbe: Enable L2 header split in 82599

This enables L2 header split when packet split is enabled for 82599.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>