Fix crash on exit on Kobo devices with no frontlight (#3221)

frontend/device/kobo/powerd.lua

@@ -87,6 +87,7 @@ function KoboPowerD:init()
 end
 
 function KoboPowerD:saveSettings()
+    if self.device.hasFrontlight() then
         -- Store BasePowerD values into settings (and not our hw_intensity, so
         -- that if frontlight was toggled off, we save and restore the previous
         -- untoggled intensity and toggle state at next startup)
@@ -111,6 +112,7 @@ function KoboPowerD:saveSettings()
             end
         end
     end
+end
 
 function KoboPowerD:frontlightIntensityHW()
     return self.hw_intensity