Something like this for quick example:
tc qdisc del dev br0 root
tc qdisc add dev br0 root handle 1: cbq avpkt 1000 bandwidth 10mbit
tc class add dev br0 parent 1: classid 1:1 cbq rate 700kbit allot 1500 prio 5 bounded isolated
tc filter add dev br0 parent 1: protocol ip prio 16 u32 match ip dst 192.168.1.133 flowid 1:1
tc filter add dev br0 parent 1: protocol ip prio 16 u32 match ip src 192.168.1.133 flowid 1:1
Substitute 700kbit for limit you want to enforce, and 192.168.1.133 for IP you want to limit. Note that 10mbit in the first line should be near your real link speed for this to function best.
Enter it in ssh console for test. When you're happy with it, in dd-wrt put it under Administration / Commands / Save Firewall to have it start automatically on reboot / interface changes.