s390/zcrypt_card: 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
   1019	    160	      0	   1179	    49b	drivers/s390/crypto/zcrypt_card.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1083	     96	      0	   1179	    49b	drivers/s390/crypto/zcrypt_card.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/crypto/zcrypt_card.c

@@ -98,7 +98,7 @@ static struct attribute *zcrypt_card_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group zcrypt_card_attr_group = {
+static const struct attribute_group zcrypt_card_attr_group = {
 	.attrs = zcrypt_card_attrs,
 };