s390/eadm_sch: improve quiesce handling

When quiescing an eadm subchannel make sure that outstanding IO is
cleared and potential timeout handlers are canceled.

Reviewed-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>