Lora: Add clear method to LoRaWANTimer.

This function must be called by the callback registered in init.
It clears the timer_id to prevent abusive use of the stop methods on expired timer.

connectivity/lorawan/system/LoRaWANTimer.cpp

@@ -64,3 +64,8 @@ void LoRaWANTimeHandler::stop(timer_event_t &obj)
     _queue->cancel(obj.timer_id);
     obj.timer_id = 0;
 }
+
+void LoRaWANTimeHandler::clear(timer_event_t &obj)
+{
+    obj.timer_id = 0;
+}

connectivity/lorawan/system/LoRaWANTimer.h

@@ -76,6 +76,13 @@ public:
      */
     void stop(timer_event_t &obj);
 
+    /** Clear timer state so it is not inadvertently canceled. This function
+     * must be called by the callback registered in init.
+     *
+     * @param [in] obj The structure containing the timer object parameters.
+     */
+    void clear(timer_event_t &obj);
+
 private:
     events::EventQueue *_queue;
 };