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.