r8169: don't configure csum function per chip version

We don't have to configure the csum function per chip (sub-)version.
The distinction is simple, versions RTL8102e and from RTL8168c onwards
support csum_v2.

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