s390/cio: check the subchannel validity for dev_busid

Check the validity of subchanel before reading other fields in
the schib.

Fixes: d3683c0 ("s390/cio: add dev_busid sysfs entry for each subchannel")
CC: <stable@vger.kernel.org>
Reported-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Link: https://lore.kernel.org/r/20211105154451.847288-1-vneethv@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: vneethv

drivers/s390/cio/css.c

@@ -437,8 +437,8 @@ static ssize_t dev_busid_show(struct device *dev,
 	struct subchannel *sch = to_subchannel(dev);
 	struct pmcw *pmcw = &sch->schib.pmcw;
 
-	if ((pmcw->st == SUBCHANNEL_TYPE_IO ||
-	     pmcw->st == SUBCHANNEL_TYPE_MSG) && pmcw->dnv)
+	if ((pmcw->st == SUBCHANNEL_TYPE_IO && pmcw->dnv) ||
+	    (pmcw->st == SUBCHANNEL_TYPE_MSG && pmcw->w))
 		return sysfs_emit(buf, "0.%x.%04x\n", sch->schid.ssid,
 				  pmcw->dev);
 	else