igb: Enable EEE LP advertisement

On EEE-capable devices, query the PHY to determine what the link partner is
advertising.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>