s390/qeth: fix error handling in checksum cmd callback

Make sure to check both return code fields before processing the
response. Otherwise we risk operating on invalid data.

Fixes: c9475369 ("s390/qeth: rework RX/TX checksum offload")
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>