s390/raw3270: 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.

File size before:
   text	   data	    bss	    dec	    hex	filename
   8069	    816	     16	   8901	   22c5	drivers/s390/char/raw3270.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   8133	    752	     16	   8901	   22c5	drivers/s390/char/raw3270.o
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/raw3270.c

@@ -1082,7 +1082,7 @@ static struct attribute * raw3270_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group raw3270_attr_group = {
+static const struct attribute_group raw3270_attr_group = {
 	.attrs = raw3270_attrs,
 };