change rtls device as field

src/beacon_positioning/src/tracker_position.cpp

@@ -35,6 +35,8 @@ public:
     serial_port->setBaudrate(115200);
     serial_port->setTimeout(800ms);
 
+    rtls_device = terabee::RtlsDevice(serial_port);
+
     RCLCPP_INFO(this->get_logger(), "Opening serial port of tracker!");
     serial_port->open();
 
@@ -115,6 +117,7 @@ private:
   terabee::RtlsDevice::config_t device_configuration;
   terabee::RtlsDevice::OnTrackerDataCallback tracker_data_callback_;
   terabee::RtlsDevice::tracker_msg_t tracker_msg;
+  terabee::RtlsDevice rtls_device;
 };
 
 int main(int argc, char **argv)