ibmveth: Fix checksum offload failure handling

Fix a number of issues in ibmveth_set_csum_offload:

- set_attr6 and clr_attr6 may be used uninitialised

- We store the result of the IPV4 checksum change in ret but overwrite
  it in a couple of places before checking it again later. Add ret4
  to make it obvious what we are doing.

- We weren't clearing the NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM flags
  if the enable of that hypervisor feature failed.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: David S. Miller <davem@davemloft.net>