improve host serial drivers

- tuh_control_xfer() update xfer result to user_data if complete
callback = NULL (sync/blocking)
- refactor host serial driver for acm/ftdi/cp210x

src/class/cdc/cdc_host.h

@@ -134,7 +134,11 @@ bool tuh_cdc_read_clear (uint8_t idx);
 
 //--------------------------------------------------------------------+
 // Control Endpoint (Request) API
-// Each Function will make a USB transfer request to/from device
+// Each Function will make a USB control transfer request to/from device
+// - If complete_cb is provided, the function will return immediately and invoke
+// the callback when request is complete.
+// - If complete_cb is NULL, the function will block until request is complete.
+// In this case, user_data should be pointed to xfer_result_t to hold the transfer result.
 //--------------------------------------------------------------------+
 
 // Request to Set Control Line State: DTR (bit 0), RTS (bit 1)