add fix so that if it's busy we wait and try again...

stm32/usb/usbd_cdc_if.c

@@ -315,7 +315,13 @@ void usb_printf(const char *format, ...)
     va_start(args, format);
     length = vsnprintf((char *)UserTxBufferFS, APP_TX_DATA_SIZE, (char *)format, args);
     va_end(args);
-    CDC_Transmit_FS(UserTxBufferFS, length);
+    for (;;) {
+        if (CDC_Transmit_FS(UserTxBufferFS, length) == USBD_BUSY) {
+            HAL_Delay(1);
+            continue;
+        }
+        break;
+    }
 }
 /* USER CODE END PRIVATE_FUNCTIONS_IMPLEMENTATION */