igb: update to allow reading/setting MDI state

This is the implementation for igb to allow forcing MDI state
via ethtool, allowing users to work around some improperly
behaving switches.

Forcing in this driver is for now only allowed when auto-neg is
enabled.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
CC: 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>