net/mlx5e: Avoid WARN_ON when configuring MQPRIO with HTB offload enabled

In the Linux kernel, the following vulnerability has been resolved: net/mlx5e: Avoid WARN_ON when configuring MQPRIO with HTB offload enabled When attempting to enable MQPRIO while HTB offload is already configured, the driver currently returns `-EINVAL` and triggers a `WARN_ON`, leading to an unnecessary call trace. Update the code to handle this case more gracefully by returning `-EOPNOTSUPP` instead, while also providing a helpful user message.