r8169: factor out PHY configuration to r8169_phy_config.c

Move chip-specific PHY configurations to separate source file
r8169_phy_config.c. This improves maintainability of the driver.

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