[netdrvr] tulip: Better MWI workaround for 21143 rev 65 chip errata

This patch works around the MWI bug on the DC21143 rev 65 Tulip by
ensuring that the receive buffers don't end on a cache line boundary
(as documented in the errata).

This patch is required for the MIPS based Cobalt Qube/RaQ as
supporting the extra PCI commands seems to reduce the chance of a hard
lockup between the Tulip and the PCI bridge.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>