AP_HAL_ChibiOS: return RC protocol being decoded

libraries/AP_HAL_ChibiOS/RCInput.h

@@ -50,6 +50,8 @@ public:
         return _rssi;
     }
 
+    const char *protocol() const override { return last_protocol; }
+
     void _timer_tick(void);
     bool rc_bind(int dsmMode) override;