si-audio-adapter/endpoint: do not sync() in loops, use ports-changed instead

Calling sync() in loops may end up looping forever if the operating system's scheduling allows pipewire to reply to this sync() before wireplumber's event loop has a chance to become idle.

In order for the new ports to appear, the object manager that monitors them needs to emit "objects-changed", which only ever happens in an idle callback. If the reply to sync() arrives before the idle callback, it gets prioritized and processed, which causes another sync(), and so on...

Since setting PortFormat in the adapter always changes the ports, watching for "ports-changed" feels like a better solution. Still, there is more room for improvement.