support all controllers in the intel _PCIe* Family of Gigabit Ethernet
Controllers Softwarer Developer's Manual_ rev 2.6 ref no. 21489.

measured AoE performance: up to 240MB/s on two 82573 ports using
8k jumbograms.

1. support intel 82566 mm/dm/v, 82571, 82572, 82573[ev].  tested with
intel pro 1000/pt single (82572) and 1000/pt dual and quad (82571) and
onboard 82563[ev] cards.  82566 untested.

2. track link status and link speed correctly keep stats on link speed changes
to aid debugging.

3. performance; use descriptor writeback (Td bit) instead of syncronous
mmio for tx ring sweep, correct some wrongly-set bits.

4. performance; use qlock not ilock around transmit.

5. calculate the mac as per yellow book specificaiton.

6. jumbo frame support.