r8169: improve conditional firmware loading for RTL8168d

Using constant MII_EXPANSION is misleading here because register 0x06
has a different meaning on page 0x0005. Here a proprietary PHY
parameter is read by writing the parameter id to register 0x05 on page
0x0005, followed by reading the parameter value from register 0x06.

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