8139cp: Enable offload features by default

I fixed TSO. Hardware checksum and scatter/gather also appear to be
working correctly both on real hardware and in QEMU's emulation.

Let's enable them by default and see if anyone screams...

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>