s390/dasd: constify attribute_group structures.

attribute_group are not supposed to change at runtime. All functions
working with attribute_group provided by <linux/sysfs.h> work
with const attribute_group. So mark the non-const structs as const.
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd_devmap.c

@@ -1634,7 +1634,7 @@ static struct attribute * dasd_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group dasd_attr_group = {
+static const struct attribute_group dasd_attr_group = {
 	.attrs = dasd_attrs,
 };