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: 7907f23adc18 ("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>