RDMA/siw: Fix error return code in siw_init_module()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: bdcf26bf

 ("rdma/siw: network and RDMA core interface")
Link: https://lore.kernel.org/r/20190718092710.85709-1-weiyongjun1@huawei.com
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>