r8169: change back SG and TSO to be disabled by default

There has been a number of reports that using SG/TSO on different chip
versions results in tx timeouts. However for a lot of people SG/TSO
works fine. Therefore disable both features by default, but allow users
to enable them. Use at own risk!

Fixes: 93681cd7

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