net: hns: adds limitation for debug port mtu

If mtu for debug port is set more than 1500, it may cause that packets
are dropped by ppe. So maximum value for debug port should be 1500.

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>