net: hns: add netif_carrier_off before change speed and duplex

If there are packets in hardware when changing the speed
or duplex, it may cause hardware hang up.

This patch adds netif_carrier_off before change speed and
duplex in ethtool_ops.set_link_ksettings, and adds
netif_carrier_on after complete the change.

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>