ixgbe: feature - driver to default with FC on.

In the past flow control wasn't enabled by default under the
incorrect assumption that this opened up us to a denial of
service attack.  However since any switch that forwarded flow
control would be extremely msiconfigured and/or buggy, this
concern no longer out weighs the preformance gains from
having FC enabled.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Acked-by: Mallikarjuna R Chilakala <mallikarjuna.chilakala@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>