igb: Ethtool support to enable and disable EEE

This patch allows users to enable and disable EEE using Ethtool.
It also allows users to get EEE settings, as supported by the device.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>