IB/sa: simplify return code logic for ib_nl_send_msg()

rdma_nl_multicast() returns either negative error code
or zero if succeeded. Remove unnecessary ret code checks
and reassignments.

Reviewed-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Alex Estrin <alex.estrin@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>