[netdrvr] tulip/winbond-840: don't let tulip.h symbol stomp ours

winbond-840 shares tulip.h with the tulip driver, because they share
many (but not all) of the same register definitions.

This is useful for the register definitions, but not helpful when it
comes to symbols that are shared among the tulip driver's C modules,
but not meant to be shared outside that one driver.

Thus, PKT_BUF_SZ is a symbol internal to tulip, but it was intruding
upon a similar symbol in winbond-840's namespace.  This was not a
problem as long as the two symbols had the same value, but upcoming
patches result in differing symbol values.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>