Fix regression (introduced in #9657) (#9759)

This should fix #9741.

frontend/device/devicelistener.lua

@@ -317,9 +317,17 @@ function DeviceListener:onRestart()
     self.ui.menu:exitOrRestart(function() UIManager:restartKOReader() end)
 end
 
-DeviceListener.onRequestSuspend = UIManager.suspend
-DeviceListener.onRequestReboot = UIManager.reboot
-DeviceListener.onRequestPowerOff = UIManager.powerOff
+function DeviceListener:onRequestSuspend()
+    UIManager:suspend()
+end
+
+function DeviceListener:onRequestReboot()
+    UIManager:reboot()
+end
+
+function DeviceListener:onRequestPowerOff()
+    UIManager:powerOff()
+end
 
 function DeviceListener:onExit(callback)
     self.ui.menu:exitOrRestart(callback)