net/mlx5: change error-prone code on Tx path

In the main loop of mlx5_tx_burst(), pointers/indexes are advanced at the
beginning. Therefore, those should be rolled back if checking resource
availability fails and breaks the loop. And some of them are even
redundant.

Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>