s390/qeth: fix error handling in adapter command callbacks

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

This matches an earlier fix for SETASSPARMS commands, see
commit ad3cbf61

 ("s390/qeth: fix error handling in checksum cmd callback").

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