Fix Receiver Port Detection in the Septentrio Driver

[flyingthingsintothings]

This fixes an issue which can prevent PX4 autopilot from recognizing the receiver port, sometimes leading to a failure to configure the driver or long attempts before it finally works. The current official release of PX4 autopilot is impacted by this issue. Currently work is being done on a new driver for Septentrio which would live in-tree (https://github.com/PX4/PX4-Autopilot/pull/22904), so I'm making this one against the release branch only. If it needs to go in the main branch as well, I can also make a pull request for that.