ixgbe: Enable FCoE offload when DCB is enabled for 82599

Currently, FCoE offload feature is turned on when the kernel config has
CONFIG_FCOE or CONFIG_FCOE_MODULE set. However, we really want to turn
FCoE offload on when there is FCoE traffic passing and turn it off when
it's just LAN traffic. Since FCoE depends on a lossless network provided
by DCB, this allows us to have FCoE turned on/off when user turns on DCB
using dcbtool.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>