Fix receiver lockups. When writing the RX Kick register, we need back up
on descriptor since we advanced one in the for loop. That will be the
last descriptor we haven't processed which is what we should write.
switch back to MII mode. Keep a sliding window for TX segments and when it
gets > 2/3 full, request a TX interrupt (window gets reset when the h/w TX
queue is empty). Add dv_xname to a few printfs. With the above changes,
the gem driver will now work on Macintoshes, even in GigE mode. On a 733
PowerMac G4 it gets ~355Mb/s TX and ~280Mb/s RX to/from an Alpha XP1000.
XXX mii autoselect is still flakey.
