For anyone interested: I found a thread on the LM forum which explains how to find the correct IRQ for USB device on system with the xhci_hcd bus in a more reliable way (thanks puleglot and Drumfix!):
https://linuxmusicians.com/viewtopic.php?p=122528&sid=ccef8b14e807cba3e2f91d60675080d6#p122528
Described it also in a comment to rtcirqus bug report.