RDMA/hns: Fix a bug with modifying mac address

When modifying mac address, it will trigger hns_roce_del_gid
function and can't delete the default gid matched the index
because the attribute of gid is null.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>