IB/usnic: Implement get_netdev hook

usnic's get_netdev hook for struct ib_device is missing - add it.

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Christian Benvenuti <benve@cisco.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>