[netX51] Drv_Urt does not deliver a callback for each received byte if used with USB (HW unit does not deliver IRQ per byte)

Due to a hardware limitation of the USB core it is not possible to generate an IRQ for every received character (there is no RX_FIFO_NOT_EMPTY irq event).

Workaround:
Receive all data from USB fifo in RxReadyCallback.