how to set the ethernet speed

There are many ways to skin this cat. For all of them though you will need to change the driver which can be found in

altera\kits\nios2\components\ecos\ecos-2.0\packages\devs\eth\smsc\lan91cxx\v2_0\src\if_lan91cxx.c

The current driver auto negotiates the link speed.

You could either:

1. Set the PHY register 4 Auto Negotiation Adevrtisement regiser to not advertise that 100MBps is supported

Probably best to do this just before Auto Negotiation is started on line 375

2. Change the code at line 375 which starts Aut Negotiation to set the RPCR register to select 10 MBps, to not AutoNegotiate and sed the LEDS as you desire

I think replacing the code with

put_reg(sc, LAN91CXX_RPCR, LAN91CXX_RPCR_LEDA_RX | LAN91CXX_RPCR_LEDB_LINK);

However I haven't tested it, and if it doesn't work you'll need to look at the lan91c11 datasheet to see what I missed.