ui/device: change last_brightness from float to int (#23029)

selfdrive/ui/ui.h

@@ -192,7 +192,7 @@ private:
   int awake_timeout = 0;
   float accel_prev = 0;
   float gyro_prev = 0;
-  float last_brightness = 0;
+  int last_brightness = 0;
   FirstOrderFilter brightness_filter;
 
   QTimer *timer;