co2_sensor: co2 is unsigned, so cannot be negative

ampel-firmware/co2_sensor.cpp

@@ -235,10 +235,10 @@ namespace sensor {
       temperature = scd30.getTemperature();
       humidity = scd30.getHumidity();
 
-      if (co2 <= 0) {
+      if (co2 == 0) {
         // NOTE: Data is available, but it's sometimes erroneous: the sensor outputs
         // zero ppm but non-zero temperature and non-zero humidity.
-        Serial.println(F("Invalid sensor data - CO2 concentration <= 0 ppm"));
+        Serial.println(F("Invalid sensor data - CO2 concentration supposedly 0 ppm"));
         switchState(BOOTUP);
       } else if ((current_state == PREPARE_CALIBRATION_UNSTABLE) || (current_state == PREPARE_CALIBRATION_STABLE)) {
         // Check for pre-calibration states first, because we do not want to