ignore device_num, connect to default sensor if no serial or usb is d…

…efined

Related to:
- orbbec#42

src/ob_camera_node_driver.cpp

@@ -89,7 +89,7 @@ void OBCameraNodeDriver::init() {
 
 std::shared_ptr<ob::Device> OBCameraNodeDriver::selectDevice(
     const std::shared_ptr<ob::DeviceList>& list) {
-  if (device_num_ == 1) {
+  if (serial_number_.empty() && usb_port_.empty()) {
     ROS_INFO_STREAM("Connecting to the default device");
     return list->getDevice(0);
   }