RDMA/hns: Limit the size of extend sge of sq

The hip08 split two hardware version. The version id are 0x20 and 0x21
according to the PCI revison. The max size of extend sge of sq is limited
to 2M for 0x20 version and 8M for 0x21 version. It may be exceeded to 2M
according to the algorithm that compute the product of wqe count and
extend sge number of every wqe. But the product always less than 8M.

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