I haven't FTDI, but I have CH340 and CP2102. They are USB to serial converter too. I think they have same characteristic. I found their interface descriptor are vendor specific. Their bInterfaceClass is 0xff. They need driver suport. I guess FTDI is same with them. You can set a breakpoint in USB_HostCdcEvent() in host_cdc.c, observe hostInterface->interfaceDesc->bInterfaceClass value. If it is 0xff, then the interface descriptor is also vendor specific. You have to modify the demo code to fit for the chip.