r8169: fix offloaded tx checksum for small packets.

8168evl offloaded checksums are wrong since commit
e5195c1f ("r8169: fix 8168evl frame padding.")
pads small packets to 60 bytes (without ethernet checksum). Typical symptoms
appear as UDP checksums which are wrong by the count of added bytes.

It isn't worth compensating. Let the driver checksum.

Due to the skb length changes, TSO code is moved before the Tx descriptor gets
written.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>