igb: display a warning message when SmartSpeed works

Current igb driver doesn't tell nothing when Link Speed is downgraded due to
SmartSpeed.  As a result, users suspect that there is something wrong with
NIC.  If the cause of it is SmartSpeed, there is no means to replace NIC. This
patch make igb notify users that SmartSpeed worked.

Signed-off-by: Koki Sanagi <sanagi.koki@jp.fujitsu.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>