net: hns: fix the bug about mtu setting

In chip V1, the maximum mtu value is 9600. But in chip V2, it is 9728.
And it is always configurates as 9600 before this patch.

Signed-off-by: Kejian Yan <yankejian@huawei.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>