AP_Airspeed: make C_TO_KELVIN a function macro; create KELVIN_TO_C

These are in celsius

libraries/AP_Airspeed/AP_Airspeed_SDP3X.cpp

@@ -237,7 +237,7 @@ float AP_Airspeed_SDP3X::_correct_pressure(float press)
         temperature = 25;
     }
 
-    float rho_air = baro_pressure / (ISA_GAS_CONSTANT * (temperature + C_TO_KELVIN));
+    float rho_air = baro_pressure / (ISA_GAS_CONSTANT * C_TO_KELVIN(temperature));
     if (!is_positive(rho_air)) {
         // bad pressure
         return press;

libraries/AP_Airspeed/AP_Airspeed_UAVCAN.cpp

@@ -121,7 +121,7 @@ void AP_Airspeed_UAVCAN::handle_airspeed(AP_UAVCAN* ap_uavcan, uint8_t node_id,
         driver->_pressure = cb.msg->differential_pressure;
         if (!isnan(cb.msg->static_air_temperature) &&
             cb.msg->static_air_temperature > 0) {
-            driver->_temperature = cb.msg->static_air_temperature - C_TO_KELVIN;
+            driver->_temperature = KELVIN_TO_C(cb.msg->static_air_temperature);
             driver->_have_temperature = true;
         }
         driver->_last_sample_time_ms = AP_HAL::millis();