QmlDesigner: Disable Style combobox for MCUs

Task-number: QDS-10338
Change-Id: I6e8242db0bd3f0981bb4d6d699f8780ae1cb6925
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

share/qtcreator/qmldesigner/statusbar/Main.qml

@@ -89,9 +89,9 @@ Item {
                 model: backend.styles
                 onActivated: backend.setCurrentStyle(styles.currentIndex)
                 openUpwards: true
-                enabled: backend.isInDesignMode
+                enabled: backend.isInDesignMode && !backend.isMCUs
                 property int currentStyleIndex: backend.currentStyle
-                onCurrentStyleIndexChanged: currentIndex = backend.currentStyle
+                onCurrentStyleIndexChanged: styles.currentIndex = backend.currentStyle
             }
         }
     }