net/mlx5: cleanup memory barriers

net/mlx5: cleanup memory barriers

Updating a consumer index to HW doesn't require a memory barrier in case
that there's no updated data to be posted to HW, but a compiler barrier
is sufficient. rte_wmb() is replaced with rte_io_wmb() when it makes
changes visible to HW, not other core.

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>