Fixes that the firmware update kan be called from the add machine -> add UMO wizard

fixes #CURA-256

plugins/USBPrinting/USBPrinterManager.py

@@ -84,6 +84,7 @@ class USBPrinterManager(QObject, SignalEmitter, OutputDevicePlugin, Extension):
 
         self._firmware_view.show()
 
+    @pyqtSlot()
     def updateAllFirmware(self):
         self.spawnFirmwareInterface("")
         for printer_connection in self._printer_connections:

resources/qml/WizardPages/UpgradeFirmware.qml

@@ -62,9 +62,7 @@ Item
             anchors.top: parent.top
             anchors.left: parent.left
             text: catalog.i18nc("@action:button","Upgrade to Marlin Firmware");
-            onClicked: {
-                printer_connection.updateAllFirmware
-            }
+            onClicked: UM.USBPrinterManager.updateAllFirmware()
         }
         Button {
             id: skipUpgradeButton