ibmveth: Remove duplicate checksum offload setup code

Remove code in the device probe function where we set up the checksum
offload feature and replace it with a call to an existing function that
is doing the same.  This is done to clean up the driver in preparation
of adding IPv6 checksum offload support.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Signed-off-by: Santiago Leon <santil@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>