[usb-audio]Remove more usage of req->node

With the recent changes to usb stack, each driver in the usb stack has its own list_node_t*
so as to avoid stamping on other driver's lists. This changeset moves
audio drivers away from using the public node. Once all
references are removed the public list_node_t pointer will be removed.

Test: fx full-build. fx serve on pixelbook.
Plugged in a usb audio device.
audio -d 1 info.
audio -d 1 tone.

Change-Id: Ic46f82acb58074a54dd85018f99a1c357f619bc9