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Yes, there is not usb2 in /proc/interrupts but a
cat "/proc/asound/card"$i"/usbbus" | rev | cut -b5
with the right value in $i
is giving 2

The same searching /sys directory with
find /sys/devices -type d -name "usb2"

Since rtirq is using /proc/interrupts, it cannot connect usb2 to the right device.
Probably this is correct, but it seems to me that the usb(x) method is not generally applicable.