Fixed App_Callback() to be interrupt safe for asynchronous TX/RX streams...

It looks like the original code was written for a sequential TX => RX => TX... chain. On the other hand, if the MCU (CDC_DEVICE) and host communicate with asynchronous TX/RX streams, the RX endpoint FSM on the MCU can become corrupted. This corruption stalls the incoming RX stream. The changes resolve this issue.