[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>

drivers/s390/cio/chsc_sch.c

@@ -61,7 +61,7 @@ static void chsc_subchannel_irq(struct subchannel *sch)
 	}
 	private->request = NULL;
 	memcpy(&request->irb, irb, sizeof(*irb));
-	stsch(sch->schid, &sch->schib);
+	cio_update_schib(sch);
 	complete(&request->completion);
 	put_device(&sch->dev);
 }