net/mlx5: LAG, Use affinity type enumerators

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

Fixes: 544fe7c2

 ("net/mlx5e: Activate HW multipath and handle port affinity based on FIB events")
Signed-off-by: Erez Alfasi <ereza@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>