AP_HAL: allow F4Light to call delay callback

libraries/AP_HAL/Scheduler.h

@@ -83,8 +83,6 @@ public:
      */
     virtual void restore_interrupts(void *) {}
 
-protected:
-
     // called by subclasses when they need to delay for some time
     virtual void call_delay_cb();
     uint16_t _min_delay_cb_ms;