add tud_speed_get()

- define both fs and hs configuration descriptor
- rename CFG_TUD_CDC_EPSIZE to CFG_TUD_CDC_EP_BUFSIZE with default size of 64 for FS, and 512 for HS

src/device/usbd.c

@@ -294,6 +294,11 @@ void usbd_driver_print_control_complete_name(bool (*control_complete) (uint8_t,
 //--------------------------------------------------------------------+
 // Application API
 //--------------------------------------------------------------------+
+tusb_speed_t tud_speed_get(void)
+{
+  return (tusb_speed_t) _usbd_dev.speed;
+}
+
 bool tud_mounted(void)
 {
   return _usbd_dev.configured;