DPL: use vedirect isdatavalid (#283)

* DPL: use VeDirect.isDataValid()

in case the communication to the Victron charger is disrupted, the
respective values in VeDirect.veFrame are not invalidated such that we
would notice a problem. instead, isDataValid() should be used to make
sure that the Victron charger is actually alive and that we can trust to
use the reported values.

* DPL: simplify canUseDirectSolarPower return statement

src/PowerLimiter.cpp

@@ -216,16 +216,12 @@ bool PowerLimiterClass::canUseDirectSolarPower()
     CONFIG_T& config = Configuration.get();
 
     if (!config.PowerLimiter_SolarPassThroughEnabled
-            || !config.Vedirect_Enabled) {
+            || !config.Vedirect_Enabled
+            || !VeDirect.isDataValid()) {
         return false;
     }
 
-    if (VeDirect.veFrame.PPV < 20) {
-        // Not enough power
-        return false;
-    }
-
-    return true;
+    return VeDirect.veFrame.PPV >= 20; // enough power?
 }