avoid flushing tx buffer on connection

Signed-off-by: Zixun LI <admin@hifiphile.com>

src/class/cdc/cdc_device.c

diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index 56d4aee..ed050ad 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -323,9 +323,7 @@
           tu_edpt_stream_t *stream_tx = &p_cdc->tx_stream;
           tu_edpt_stream_open(stream_tx, rhport, desc_ep, CFG_TUD_CDC_TX_EPSIZE);
 
-  #if CFG_TUD_CDC_TX_PERSISTENT
-          tu_edpt_stream_write_xfer(stream_tx); // flush pending data
-  #else
+  #if !CFG_TUD_CDC_TX_PERSISTENT
           tu_edpt_stream_clear(stream_tx);
   #endif
         } else {