igb: fix warning if !CONFIG_IGB_HWMON

Fix warning about code defined but never used if IGB_HWMON not defined.
Reported-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/e1000_82575.c

@@ -2711,6 +2711,7 @@ static const u8 e1000_emc_therm_limit[4] = {
 	E1000_EMC_DIODE3_THERM_LIMIT
 };
 
+#ifdef CONFIG_IGB_HWMON
 /**
  *  igb_get_thermal_sensor_data_generic - Gathers thermal sensor data
  *  @hw: pointer to hardware structure
@@ -2833,6 +2834,7 @@ static s32 igb_init_thermal_sensor_thresh_generic(struct e1000_hw *hw)
 	return status;
 }
 
+#endif
 static struct e1000_mac_operations e1000_mac_ops_82575 = {
 	.init_hw              = igb_init_hw_82575,
 	.check_for_link       = igb_check_for_link_82575,