net/mlx5: Lag, move lag files into directory

Downstream patches add another lag related file so it makes
sense to have all the lag files in a dedicated directory.

Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>