net/mlx5: Clear LAG notifier pointer after unregister

After returning from unregister_netdevice_notifier_dev_net(), set the
notifier_call field to NULL so successive call to mlx5_lag_add() will
function as expected.

Fixes: 7907f23a

 ("net/mlx5: Implement RoCE LAG feature")
Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Raed Salem <raeds@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>