net/mlx5: fix Tx completion descriptors fetching loop

net/mlx5: fix Tx completion descriptors fetching loop

This patch limits the amount of fetched and processed
completion descriptors in one tx_burst routine call.

The completion processing involves the buffer freeing
which may be time consuming and introduce the significant
latency, so limiting the amount of processed completions
mitigates the latency issue.

Fixes: 18a1c20044c0 ("net/mlx5: implement Tx burst template")

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>