s390/qeth: fix error handling for isolation mode cmds

commit e2dfcfba upstream.

Current(?) OSA devices also store their cmd-specific return codes for
SET_ACCESS_CONTROL cmds into the top-level cmd->hdr.return_code.
So once we added stricter checking for the top-level field a while ago,
none of the error logic that rolls back the user's configuration to its
old state is applied any longer.

For this specific cmd, go back to the old model where we peek into the
cmd structure even though the top-level field indicated an error.

Fixes: 686c97ee

 ("s390/qeth: fix error handling in adapter command callbacks")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>