net/mlx5: Change ownership model for lag

Lag is used to combine two PCI functions of the same HCA into a single
logical unit. This is a core functionality and as such should be managed by
the core driver. Currently this isn't the case. While we store the lag
software structure inside the lower device, its lifetime (creation /
destruction) is dictated by the mlx5e part. Change the ownership model so
lag is tied to the lifetime of the lower level driver instead to the
mlx5e part.

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