multiple cameras - device_num logic unnecessary

[bmegli]

Currently when using multiple sensors user has to pass number of sensors to driver

• this is the only place in code I can see device_num used

This is unnecessary and difficult for user (what if sensors are added dynamically to the system by user?)

It should be enough to:

• check if serial and USB descriptor are both empty (not defined)
  • connect to default sensor in that case
• connect by serial otherwise
• connect by USB descriptor otherwise

Link to relevant code fragment:

https://github.com/orbbec/OrbbecSDK_ROS1/blob/c587b26c7c0191af5006ec76a8a32945f307c324/src/ob_camera_node_driver.cpp#L92-L114

[bmegli]

Example implementation in:

• https://github.com/Extend-Robotics/OrbbecSDK_ROS1/commit/07daf95ed28dd8c3afa1ddba6afd95ed84c92cea