I noticed that Viro's "use alloc_etherdev" patch left a
small problem:  only "usbnet" did SET_NETDEV_DEV, and
that used the wrong driver model device.

This has all the usb network drivers use SET_NETDEV_DEV,
so the /sys/class/net/*/{device,driver} files link to the
relevant usb interface and to its device driver.