IB/core: fix an error code in ib_core_init()

We should return the error code if ib_add_ibnl_clients() fails.  The
current code returns success.

Fixes: 735c631a

 ('IB/core: Register SA ibnl client during ib_core initialization')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>