ibmvnic: enable RX checksum offload

Enable RX Checksum offload feature in the ibmvnic driver.

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