Is it possible switch port 5 to 100FD mode on dg834gt with bcm6348 cpu ?

Command like this doesn't work:  robocfg port 5 media 100FD

root@OpenWrt:/# robocfg show
probing eth1
Switch: enabled
Port 0(W):  DOWN enabled stp: none vlan: 0 mac: 00:00:00:00:00:00
Port 1(4):  DOWN enabled stp: none vlan: 0 mac: 00:00:00:00:00:00
Port 2(3):  DOWN enabled stp: none vlan: 1 mac: 00:00:00:00:00:00
Port 3(2): 100FD enabled stp: none vlan: 1 mac: 00:00:00:00:00:00
Port 4(1):  DOWN enabled stp: none vlan: 1 mac: 00:00:00:00:00:00
Port 5(C):  10HD enabled stp: none vlan: 1 mac: 00:00:00:00:00:00
VLANs: BCM5325/535x enabled mac_check mac_hash
vlan0: 0 1 5t
vlan1: 2 3 4 5t

thanks