net/mlx5: Lag, Use mlx5_lag_dev() instead of derefering pointers

Use the existing wrapper mlx5_lag_dev() to access the lag object from
dev for better maintainability and consistent code.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>