net/mlx5: Align flow steering allocation namespace to common style

Flow steering is a low level internal driver API, as such it relies on
the callers to check if namespace is supported and not rely on some
compilation flag.

Link: https://lore.kernel.org/r/cfb411a8a9ed2a1471810af254bdc0f03469f79c.1649232994.git.leonro@nvidia.com


Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>