- Don't claim 7422 MTU size is support by various 8111/8169 chips (PCI devices);

- Don't claim 7422 MTU size is support by various 8111/8169 chips (PCI devices);
  6144 MTU size works reliably.
    Set MTU above 6144 (6 * 1024) on these chips and do following test:
    netperf -H host -l 30 -t UDP_STREAM -- -m (mtu-28)
    All kinds of wiredness will pop up on the test box.
- Set max supported MTU to 9216 for 8168D.
  Obtained-from: Realtek r8168-8.008.00
- Set max supported MTU to 6144 for non-8168D GigE chips.
- Cleanup jumbo frame/MTU size related macros.

# As usual, 8169(with 88E1000 PHY) does not seem to work well with any jumbo
# frame size