/etc/conf.modules:

  alias eth0 eepro100
  options eepro100 options=0x30
                            
See parameters at http://www.scyld.com/network/eepro100.html

And for 3c59x (PSI stock):

  alias eth0 3c59x
  options eth0 options=20 full_duplex=1

see: http://www.scyld.com/network/vortex.html