[drivers/serial]resolve compile error

Signed-off-by: WillianChan <chentingwei@rt-thread.com>

components/drivers/serial/serial.c

@@ -328,6 +328,7 @@ rt_inline int _serial_int_tx(struct rt_serial_device *serial, const rt_uint8_t *
                 rt_completion_wait(&(tx->completion), RT_WAITING_FOREVER);
                 continue;
             }
+        }
 
         if (serial->ops->putc(serial, *(char*)data) == -1)
         {