net: hns: set-coalesce-usecs returns errno by dsaf.ko

It may fail to set coalesce usecs to HW, and Ethtool needs to know if it
is successful to cfg the parameter or not. So it needs return the errno by
dsaf.ko.

Signed-off-by: Lisheng <lisheng011@huawei.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>