To go back to the Linksys firmware it's necessary to use the "TFTP method"

Download the Linksys TFTP utility for Windows from:

http://www.dd-wrt.com/dd-wrtv2/downloads/others/tornado/Windows-TFTP/tftp.exe

also download the correct firmware file from the Linksys website.

Set your PC IP to 192.168.1.2

In the Linksys utility set the "server" to 192.168.1.1   (The router always uses this IP during boot)

Browse for the file to upload.

A few seconds after you turn on the router click "upgrade" you will need to try a few times to get the timing correct.

Once you get "Upgrade Sucessful" then wait for the power light to stop flashing, then reboot the router and you'll be good.