net/mlx5: LAG, Use port enumerators

Instead of using explicit array indexes, simply use
ports enumerators to make the code more readable.

Fixes: 7907f23a

 ("net/mlx5: Implement RoCE LAG feature")
Signed-off-by: Erez Alfasi <ereza@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>