s390/qeth: extend Checksum Offload Assists for IPv6

Add some wrappers to make the protocol-specific Assist code a little
more generic, and use them for sending protocol-agnostic commands in
the Checksum Offload Assist code.

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>