Diag: improve E255 handling:

- allways run through finishDiag() method
 - on finishing diag: only remove elements in previousMachineEventSet if
data is valid

src/ATBAPP/DeviceControllerDiag.h

@@ -170,7 +170,7 @@ private slots:
     void private_setDiagEvent(DeviceController::State result);
     void private_sendDiagEvent(DCMachineEvent result, DeviceController::Action action);
 
-    void private_finishDiag();
+    void private_finishDiag(bool dataValid = false);
 
     void sys_superviseSystem();