tulip: fix 21142 with 10Mbps without negotiation

with current kernels, tulip 21142 ethernet controllers fail to connect
to a 10Mbps only (i.e. without negotiation-partner) network.  It used
to work in 2.4 kernels.  Fix that.  Tested on a 21142 Rev 0x11.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: David S. Miller <davem@davemloft.net>