Stability fixes:

Stability fixes:

- In wb_rxeof(), if the received packet is less than MINCLSIZE bytes,
  copy it to an mbuf chain so as to be more frugal in our use of mbuf
  clusters.

- The Winbond chip, like the ASIX, wants the 'TX interrupt request'
  bit set in the _first_ fragment of a transmitted frame, not the
  last. (At least the Winbond manual states this unambiguously; too
  bad I wasn't paying attention when I read it the first time.)

- Turn off the transmit threshold mechanism (initialize the threshold
  to 0). This effectively puts the chip in 'store and forward' mode
  which seems to cut down on transmit errors a little. It may also
  reduce transmit performace a bit, but I'm willing to do that if it
  means better reliability.