RDMA/hns: Use a separated function for setting extend sge paramters

Moves the related lines of setting extended sge size into a separate
function as well as remove the unused variables.

Link: https://lore.kernel.org/r/1562593285-8037-5-git-send-email-oulijun@huawei.com


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