ibmvnic: Fix IP offload control buffer

Set some missing fields in the IP control offload buffer. This buffer is
used to enable checksum and TCP segmentation offload in the VNIC server.
The buffer length field and the checksum offloading bits were not set
properly, so fix that here.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>