r8169: fix wrong PHY ID issue with RTL8168dp

As reported in [0] at least one RTL8168dp version has problems
establishing a link. This chip version has an integrated RTL8211b PHY,
however the chip seems to report a wrong PHY ID, resulting in a wrong
PHY driver (for Generic Realtek PHY) being loaded.
Work around this issue by adding a hook to r8168dp_2_mdio_read()
for returning the correct PHY ID.

[0] https://bbs.archlinux.org/viewtopic.php?id=246508

Fixes: 242cd9b5

 ("r8169: use phy_resume/phy_suspend")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>