r8169: add workaround for RTL8168evl TSO hw issues

Add workaround for hw issues with TSO on RTL8168evl. This workaround is
based on information I got from Realtek, and *should* allow to safely
enable TSO on this chip version.

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