net: hns3: refine the handle for hns3_nic_net_open/stop()

When triggering nic down, there is a time window between bringing down
the protocol stack and stopping the work task. If the net is up in the
time window, it may bring up the protocol stack again.

This patch fixes it by stop the work task at the beginning of
hns3_nic_net_stop(). To keep symmetrical, start the work task at the
end of hns3_nic_net_open().

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>