[IB] umad: two small fixes

Two small fixes for the umad module:
 - set kobject name for issm device properly
 - in ib_umad_add_one(), s is subtracted from the index i when
   initializing ports, so s should be subtracted from the index when
   freeing ports in the error path as well.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>