r8169: improve RTL8168d PHY initialization

Certain parts of the PHY initialization are the same for sub versions
1 and 2 of RTL8168d. So let's factor this out to simplify the code.

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