net: hns3: fix shaper parameter algorithm

Currently when hns3 driver configures the tm shaper to limit
bandwidth below 20Mbit using the parameters calculated by
hclge_shaper_para_calc(), the actual bandwidth limited by tm
hardware module is not accurate enough, for example, 1.28 Mbit
when the user is configuring 1 Mbit.

This patch adjusts the ir_calc to be closer to ir, and
always calculate the ir_b parameter when user is configuring
a small bandwidth. Also, removes an unnecessary parenthesis
when calculating denominator.

Fixes: 84844054 ("net: hns3: Add support of TX Scheduler & Shaper to HNS3 driver")
Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>