RDMA/core: Fix return error value in _ib_modify_qp() to negative

The error codes in _ib_modify_qp() are supposed to be negative errno.

Fixes: 7a5c938b ("IB/core: Check for rdma_protocol_ib only after validating port_num")
Link: https://lore.kernel.org/r/1595645787-20375-1-git-send-email-liheng40@huawei.com


Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Li Heng <liheng40@huawei.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>