[S390] dcssblk: revert devt conversion

git commit f331c029 changed users of
->first_minor to devt. This broke device handling in dcssblk, so that
no additional devices could be added after the first one.

This patch reverts the devt conversion to the previous ->first_minor
handling.
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dcssblk.c

@@ -127,7 +127,7 @@ dcssblk_assign_free_minor(struct dcssblk_dev_info *dev_info)
 		found = 0;
 		// test if minor available
 		list_for_each_entry(entry, &dcssblk_devices, lh)
-			if (minor == MINOR(disk_devt(entry->gd)))
+			if (minor == entry->gd->first_minor)
 				found++;
 		if (!found) break; // got unused minor
 	}
@@ -625,7 +625,7 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char
 	if (rc)
 		goto release_gd;
 	sprintf(dev_info->gd->disk_name, "dcssblk%d",
-		MINOR(disk_devt(dev_info->gd)));
+		dev_info->gd->first_minor);
 	list_add_tail(&dev_info->lh, &dcssblk_devices);
 
 	if (!try_module_get(THIS_MODULE)) {