igb: implement 580 MDI setting support

In order for igb to support MDI setting support via
ethtool this code is needed to allow setting the MDI state
via software.

This is in regards to the related ethtool patch

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>