[S390] cio: introduce cio_update_schib

There is the chance that we get condition code 0 for a stsch but
the resulting schib is not vaild. In the current code there are
2 cases:
* we do a check for validity of the schib after stsch, but at this
  time we have already stored the invaild schib in the subchannel
  structure. This may lead to problems.
* we don't do a check for validity, which is not that good either.

The patch addresses both issues by introducing the stsch wrapper
cio_update_schib which performs stsch on a local schib. This schib
is only written back to the subchannel if it's valid.

side note: For some functions (chp_events) the return codes are
different now (-ENXIO vs -ENODEV) but this shouldn't do harm
since the caller doesn't check for _specific_ errors.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/drivers/s390/cio/cmf.c b/drivers/s390/cio/cmf.c
index a90b28c..288482b 100644
--- a/drivers/s390/cio/cmf.c
+++ b/drivers/s390/cio/cmf.c
@@ -195,7 +195,8 @@
 	/* msch can silently fail, so do it again if necessary */
 	for (retry = 0; retry < 3; retry++) {
 		/* prepare schib */
-		stsch(sch->schid, schib);
+		if (cio_update_schib(sch))
+			return -ENODEV;
 		schib->pmcw.mme  = mme;
 		schib->pmcw.mbfc = mbfc;
 		/* address can be either a block address or a block index */
@@ -219,7 +220,8 @@
 				ret = -EINVAL;
 				break;
 		}
-		stsch(sch->schid, schib); /* restore the schib */
+		if (cio_update_schib(sch))
+			return -ENODEV;
 
 		if (ret)
 			break;
@@ -338,7 +340,7 @@
 
 	sch = to_subchannel(cdev->dev.parent);
 
-	if (stsch(sch->schid, &sch->schib))
+	if (cio_update_schib(sch))
 		return -ENODEV;
 
 	if (scsw_fctl(&sch->schib.scsw) & SCSW_FCTL_START_FUNC) {