r8169: replace mii_bus member with phy_device member in struct rtl8169_private

Accessing the phy_device indirectly via the netdevice causes few issues:
- Accessing the phy_device when it's not attached may cause a NPE.
- If we have to access the phy_device when it's not attached we have
  to use mdiobus_get_phy() to get a reference to the phy_device.

Therefore store a phy_device reference in struct rtl8169_private directly.

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