r8169: create function pointer array for PHY init functions

Using a function pointer array makes this easier to read and better
maintainable. AFAIK function pointer arrays cause some performance
drawback due to Spectre mitigation, but we're not in a hot path here.

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