mlx4: Rename member mlx4_en_dev.nb to netdev_nb

Rename the mlx4_en_dev.nb notifier_block member to netdev_nb in
preparation to add a mlx4 core notifier_block.

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Tested-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>