net: hns: bug fix about the overflow of mss

When set MTU to the minimum value 68, there are increasing number
of error packets occur, which is caused by the overflowed value of
mss. This patch fix the bug.

Signed-off-by: Daode Huang <huangdaode@hisilicon.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>