Debugging via JTAG is not possible in this dongle since the JTAG pins are not routed on the dongle. But UART debugging is possible with the following modification:
Put a 0 Ohm resistor between positions 2 and 3 of DPR1 and DPR2 so that the UART TX and RX are routed to the 12 pin header. Note that these TX and RX work on TTL levels.
You can refer to the schematic of FX3S Raid-on-chip USB Dongle in the following link: