mlx5: Numa domain improvements.

Properly allocate all mlx5en(4) structures from correct numa domain.

While at it cleanup unused numa domain integers deriving from the
Linux version of mlx5en(4).

Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking

(cherry picked from commit 7c3eff94bda8bb746bfa7a5edc81b014e2dc97f6)