lansenth driver: parse temperature as AutoSigned in order to see negative values

src/driver_lansenth.cc

@@ -53,7 +53,7 @@ namespace
              "The current temperature.",
              DEFAULT_PRINT_PROPERTIES,
             Quantity::Temperature,
-            VifScaling::Auto,
+            VifScaling::AutoSigned,
             FieldMatcher::build()
             .set(MeasurementType::Instantaneous)
             .set(VIFRange::ExternalTemperature)