igb: Enable hwmon data output for thermal sensors via I2C.

Some of our adapters have internal sensors that report thermal data.  This
patch enables reporting of that data via sysfs.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>