Function returns zero - make it void.

While there make struct net_device const.
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>