Add tx callback to cdc device

Useful for continuous transmission of data, which is difficult currently
because there is no notification of tx completion.

src/class/cdc/cdc_device.c

@@ -416,6 +416,9 @@ bool cdcd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint32_
   //       Though maybe the baudrate is not really important !!!
   if ( ep_addr == p_cdc->ep_in )
   {
+    // invoke transmit callback to possibly refill tx fifo
+    if ( tud_cdc_tx_cb && !tu_fifo_full(&p_cdc->tx_ff) ) tud_cdc_tx_cb(itf);
+
     if ( 0 == tud_cdc_n_write_flush(itf) )
     {
       // There is no data left, a ZLP should be sent if