net/mlx5: revert fallback to Verbs Tx allocation

On Windows, DevX CQ creation was failing because
of a different allocation fallback behaviour.

Fixes: 4abc12b9331c ("net/mlx5: fall back to Verbs Tx allocation without DevX")
Cc: stable@dpdk.org

Reported-by: Pier Damouny <pdamouny@nvidia.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Dariusz Sosnowski <dsosnowski@nvidia.com>