igb: introduce igb_thermal_sensor_event for sensor checking

The code for thermal sensor checking should be wrapped into a function.

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>