net: phy: stop PHY if needed when entering phy_disconnect

Stop PHY if needed when entering phy_disconnect. This allows drivers
that don't need a separate call to phy_stop() to omit this call.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>