s390x/ioinst: Fixed priority of operand exceptions

Operand exceptions have a lower priority than specification and address
exceptions. Thus the checks for operand exceptions must be done later.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
