net/mlx5: Refactor vport scheduling element creation function

Modify the vport scheduling element creation function to get the parent
node directly, aligning it with the group creation function.

This ensures a consistent flow for scheduling elements creation, as the
parent nodes already contain the device and parent element index.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>