Update your opkg and install this package:
opkg install hub-ctrl
Then use command to turn off the power of the USB:
hub-ctrl -h 0 -P 2 -p 0
Try connecting something chargeable through the USB port of the RUTX and then enter these commands one by one and see which one stops the charging.
hub-ctrl -h 0 -P 1 -p 0
hub-ctrl -h 0 -P 3 -p 0
hub-ctrl -h 0 -P 4 -p 0