r8169: add helper r8168d_phy_param

Integrated PHY's from RTL8168d support an indirect access method for
PHY parameters. On page 0x0005 parameter number is written to register
0x05, then the parameter can be accessed via register 0x06.
Add a helper for this to improve readability and simplify the code.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>