Fix: really trigger standby only once on DPL shutdown

relying on updateInverters() to return false after requesting inverter
standby is not a good idea. the standby function will always set a new
limit, and the updateInverters() function may fail to recognize that no
change is actually required.

instead, we trigger the standby only once and rely on the
updateInverters() function being called as part of the DPL loop() to
apply the standby state.

src/PowerLimiter.cpp

@@ -82,6 +82,10 @@ void PowerLimiterClass::announceStatus(PowerLimiterClass::Status status)
     _lastStatusPrinted = millis();
 }
 
+/**
+ * NOTE: this method relies on being called regularly, i.e., as part of the
+ * loop(), and that it is called *after* updateInverters().
+ */
 bool PowerLimiterClass::isDisabled()
 {
     auto const& config = Configuration.get();
@@ -104,7 +108,10 @@ bool PowerLimiterClass::isDisabled()
 
     for (auto& upInv : _inverters) { upInv->standby(); }
 
-    _shutdownComplete = !updateInverters();
+    // we triggered the shutdown, and we won't trigger it again until the DPL
+    // enabled and disabled again. we rely that updateInverters() is called
+    // in the DPL loop(), applying the standby limit and power state.
+    _shutdownComplete = true;
 
     return true;
 }