acenic/ga620 network driver tweaks

A slightly tuned version of the ga620 driver developed for the 3rd ed. The ga620 driver was not included in the 4th ed., but this driver also works in the 4th ed.

Files:

etherga620.c

All tests were done using 2 800Mhz PIII machines with copper acenic. 64K and 512K labels are the size of QMAX in /sys/src/9/ip/tcp.c changing QMAX does not seems to make any difference at this point.