Hi openhy,
The 88W8897 Wi-Fi module has only one radio device and dual-band is enabled by default. As shown in this page:
http://wiki.espressobin.net/tiki-index.php?page=Configuring+wireless+network+on+ESPRESSObin
you have to change option disabled 1 to option disabled 0 in your /etc/config/wireless, commit the changes and run wifi command for the changes to take effect. Or, just do:
$ uci set wireless.radio0.disabled='0' && uci commit wireless && wifi
