net: hns3: code optimization for hclge_rx_buffer_calc

There are four steps to calcuate the rx private buffer, each step
can be done in a function to avoid code duplication and aid code
readability.

This patch adds three separate functions do the job. Also, the
function name more or less make the comment redundant, so remove
some obvious comment.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>