IB/ipoib: function interface change

The ipoib_ib_dev_down/ipoib_ib_dev_stop return zero unconditionally
and the callers never check the returned values,
change the return type to void and remove the redundant return values.

Reviewed-by: Shan Hai <shan.hai@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>