DPL: address overflow issue when testing backoff

src/PowerLimiter.cpp

@@ -209,8 +209,7 @@ void PowerLimiterClass::loop()
 
     // since _lastCalculation and _calculationBackoffMs are initialized to
     // zero, this test is passed the first time the condition is checked.
-    // TODO(schlimmchen): not wrap-around safe
-    if (millis() < (_lastCalculation + _calculationBackoffMs)) {
+    if ((millis() - _lastCalculation) < _calculationBackoffMs) {
         return announceStatus(Status::Stable);
     }