r8169: don't configure max jumbo frame size per chip version

We don't have to configure the max jumbo frame size per chip
(sub-)version. It can be easily determined based on the chip family.
And new members of the RTL8168 family (if there are any) should be
automatically covered.

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