net/mlx5: poll completion queue once per a call

net/mlx5: poll completion queue once per a call

mlx5_tx_complete() polls completion queue multiple times until it
encounters an invalid entry. As Tx completions are suppressed by
MLX5_TX_COMP_THRESH, it is waste of cycles to expect multiple
completions in a poll. And freeing too many buffers in a call can
cause high jitter.
This patch improves throughput a little.

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>