net/mlx5: fix assert for Tx completion queue count

net/mlx5: fix assert for Tx completion queue count

There should be at least one Tx CQE remained if Tx WQ and txq->elts[] have
available slots to send a packet because the size of Tx CQ is exactly
calculated from the size of other resources. As it is guaranteed, it is
checked by an assertion.

max_elts is checked after the assertion for Tx CQ. If no slot is available
in txq->elts[], the assertion would be wrong.

Fixes: 2eefbec531c7 ("net/mlx5: add missing sanity checks for Tx completion queue")
Fixes: 6ce84bd88919 ("net/mlx5: add enhanced multi-packet send for ConnectX-5")
Cc: stable@dpdk.org
Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Xueming Li <xuemingl@mellanox.com>