[TG3]: Fix 5700/5701 DMA corruption on Apple G4.

Fix 5700/5701 DMA write corruption on Apple G4 by detecting the Apple
UniNorth PCI 1.5 chipset and adjusting the DMA write boundary to 16. DMA
test fails to detect the problem with this chipset.

Thanks to Manuel Perez Ayala for reporting the problem and helping to
debug it.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>