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.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Yael Chemla <ychemla@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>