r8169: add helper r8168g_phy_param

Integrated PHY's from RTL8168g support an indirect access method for
PHY parameters. On page 0x0a43 parameter number is written to register
0x13, then the parameter can be accessed via register 0x14.
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>