update usbnet some:
- move rx/tx xfer flags into usbnet_cdata
- move the callbacks into usbnet_ops structure
- move rx/tx xfer flags arguments from usbnet_init_rx_tx()
  and move them all into usbnet_attach() arguments
- s/miibus/mii/ in some places for consistency

other clean up:
- create wrapper functions for callbacks, move knowledge about
  special handling (OK to be missing, error eating) there.
- use cdata pointer if already available
- provide some more macros (will be real functions later) for
  accessing usbnet members, use existing ones more

bump kernel version.

(mrg)