net: hns: fix ethtool loopback fail bug

When run ethtool cmd(ethtool -t ethx) again and again for a long
time, it will be probabilistically fail. The PHYs' registers may
be on different pages, so it must be switch to the right page
before setting PHYs' registers.
And __lb_up() calls phy_start() to startup the PHYs device, but
this function may change Copper Control Register(Page 0, Register 0)
to an other value. It would cause phy loopback test fail. if we
remove phy_start(), we have to remove the relative phy_stop(),
phy_disconnect() when doing phy loopback to keep the phy stay in
right status.

Reported-by: hejun <hjat2005@huawei.com>
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>