net: phy: Do not warn in phy_stop() on PHY_DOWN

When phy_is_started() was added to catch incorrect PHY states,
phy_stop() would not be qualified against PHY_DOWN. It is possible to
reach that state when the PHY driver has been unbound and the network
device is then brought down.

Fixes: 2b3e88ea

 ("net: phy: improve phy state checking")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>