for example, let us assume u use e100.o.
1) compile u nic driver as module.
2) use modinfo e100 or check the src/linux/documentation/networking/xx.txt or check the kernel sourc code to see which paramters control this. for e100 is e100_speed_duplex.
3) modify the /etc/module.conf to add options for u e100.
that's all.