ibmveth: Checksum offload is always disabled

Commit b9367bf3 (net: ibmveth: convert to hw_features) reversed
a check in ibmveth_set_csum_offload that results in checksum offload
never being enabled.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org> # 3.0+
Signed-off-by: David S. Miller <davem@davemloft.net>