net: hns3: fix for ipv6 address loss problem after setting channels

The function of dev_close and dev_open is just likes ifconfig <netif> down
and ifconfig <netif> up. The ipv6 address will be lost after dev_close and
dev_open are called. This patch uses hns3_nic_net_stop to replace dev_close
and uses hns3_nic_net_open to replace dev_open.

Signed-off-by: Fuyun Liang <liangfuyun1@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>