RDMA/hns: Refactor process of setting extended sge

The variable 'cnt' is used to represent the max number of sge an SQ WQE
can use at first, then it means how many extended sge an SQ has. In
addition, this function has no need to return a value. So refactor and
encapsulate the parts of getting number of extended sge a WQE can use to
make it easier to understand.

Link: https://lore.kernel.org/r/1606558959-48510-4-git-send-email-liweihang@huawei.com


Signed-off-by: Weihang Li <liweihang@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>