r8169: remove r8169_csum_workaround

The loop in r8169_csum_workaround is called only if in
msdn_giant_send_check a copy of the skb header needs to be made and
we don't have enough memory. Let's simply drop the packet in that case
so that we can remove r8169_csum_workaround.

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