r8169: fix offloaded tx checksum for small packets.

r8169: fix offloaded tx checksum for small packets.

The workaround introduced by commit e5195c1f31f3 'r8169: fix 8168evl
frame padding.' upstream was incorrect and was entirely replaced in
commit b423e9ae49d7 'r8169: fix offloaded tx checksum for small
packets.'

On the 3.2.y branch, the first commit has effectively been applied
twice: the first time by itself, and the second time in commit
3cf40360f431 which squashed the two upstream commits together.  That
left us with both the incorrect and the correct workaround in place.
Remove the incorrect one.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Francois Romieu <romieu@fr.zoreil.com>