RDMA/hns: Fix wrong assignment of SRQ's max_wr

srq's attribute max_wr should be 1 less than the total count of wqe.

Fixes: ffb1308b

 ("RDMA/hns: Move SRQ code to the reasonable place")
Link: https://lore.kernel.org/r/1588931159-56875-5-git-send-email-liweihang@huawei.com
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>