r8169: fix performance issue on RTL8168evl

Disabling TSO but leaving SG active results is a significant
performance drop. Therefore disable also SG on RTL8168evl.
This restores the original performance.

Fixes: 93681cd7

 ("r8169: enable HW csum and TSO")
Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>