ixgbe: Allow link flow control in DCB mode for 82599 adapters

82599 supports using either link flow control or priority flow control when
in DCB mode.  The dcbnl interface already supports sending down
configurations through rtnetlink that can enable LFC when DCB is enabled,
so the driver should take advantage of this.

82598 does not support using LFC when DCB is enabled, so explicitly disable
it when we're in DCB mode.  This means we always run in PFC mode when DCB
is enabled.

Signed-off-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>