igb: Add messaging for thermal sensor events on i350 devices

This feature adds messaging to the link status change to notify
the user if the device returned from a downshift or power off
event due to the Thermal Sensor feature in i350 parts. Feature
is only available on internal copper ports.

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>