s390/qeth: add IPv6 RX checksum offload support

Check if a qeth device supports IPv6 RX checksum offload, and hook it up
into the existing NETIF_F_RXCSUM support.
As NETIF_F_RXCSUM is now backed by a combination of HW Assists, we need
to be a little smarter when dealing with errors during a configuration
change:
- switching on NETIF_F_RXCSUM only makes sense if at least one HW Assist
  was enabled successfully.
- for switching off NETIF_F_RXCSUM, all available HW Assists need to be
  deactivated.

Signed-off-by: Kittipon Meesompop <kmeesomp@linux.vnet.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>