igb: Add flow control advertising to ethtool setting.

Added pause flag for bi-directional flow control advertising to ethtool
settings.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>