usb: device_next: rename usbd_class_iter to usbd_class_node

For the last dozen commits, the role of usbd_class_node has actually
been taken over by usbd_class_iter. After cleaning up and merging
usbd_class_node and usbd_class_data, we can rename usbd_class_iter to
usbd_class_node to better reflect its role.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>