AP_EFI: make C_TO_KELVIN a function macro; create KELVIN_TO_C

These are in celsius

libraries/AP_EFI/AP_EFI.cpp

@@ -225,8 +225,8 @@ void AP_EFI::send_mavlink_status(mavlink_channel_t chan)
         state.spark_dwell_time_ms,
         state.atmospheric_pressure_kpa,
         state.intake_manifold_pressure_kpa,
-        (state.intake_manifold_temperature - C_TO_KELVIN),
-        (state.cylinder_status[0].cylinder_head_temperature - C_TO_KELVIN),
+        KELVIN_TO_C(state.intake_manifold_temperature),
+        KELVIN_TO_C(state.cylinder_status[0].cylinder_head_temperature),
         state.cylinder_status[0].ignition_timing_deg,
         state.cylinder_status[0].injection_time_ms,
         0, 0, 0);

libraries/AP_EFI/AP_EFI_NWPMU.cpp

@@ -86,12 +86,12 @@ void AP_EFI_NWPMU::handle_frame(AP_HAL::CANFrame &frame)
         memcpy(&data, frame.data, sizeof(data));
         switch((NWPMU_TEMPERATURE_TYPE)data.temp_type) {
         case NWPMU_TEMPERATURE_TYPE::C:
-            internal_state.coolant_temperature = data.coolant_temp * 0.1f + C_TO_KELVIN;
-            internal_state.cylinder_status[0].cylinder_head_temperature = data.coolant_temp * 0.1f + C_TO_KELVIN;
+            internal_state.coolant_temperature = C_TO_KELVIN(data.coolant_temp * 0.1f);
+            internal_state.cylinder_status[0].cylinder_head_temperature = C_TO_KELVIN(data.coolant_temp * 0.1f);
             break;
         case NWPMU_TEMPERATURE_TYPE::F:
-            internal_state.coolant_temperature = ((data.coolant_temp * 0.1f) - 32 * 5/9) + C_TO_KELVIN;
-            internal_state.cylinder_status[0].cylinder_head_temperature = ((data.coolant_temp * 0.1f) - 32 * 5/9) + C_TO_KELVIN;
+            internal_state.coolant_temperature = C_TO_KELVIN(((data.coolant_temp * 0.1f) - 32 * 5/9));
+            internal_state.cylinder_status[0].cylinder_head_temperature = C_TO_KELVIN(((data.coolant_temp * 0.1f) - 32 * 5/9));
             break;
         default:
             break;