net: hns: change code style from a = a + x to a += x

This patch fixes the code style in hns driver. Change it from
"buff = buff + xxx" to "buff += xxx". The reveiw comments is
from andy.

Reviewed-by: Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
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>