RDMA/core: Fix error return in _ib_modify_qp()

Fix to return error code PTR_ERR() from the error handling case instead of
0.

Fixes: 51aab126 ("RDMA/core: Get xmit slave for LAG")
Link: https://lore.kernel.org/r/20201016075845.129562-1-jingxiangfeng@huawei.com


Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>