ixgbe: Add support for NETIF_F_FCOE_MTU to 82599 devices

Enable netdev feature flag bit NETIF_F_FCOE_MTU for 82599 devices and enable
jumbo frame correspondingly when NETIF_F_FCOE_MTU is set.

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>