Function returns zero - make it void.

While there make struct net_device const.
Signed-off-by: NYuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: NParav Pandit <parav@mellanox.com>
Reviewed-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>