atm: adummy: constify attribute_group structure

Functions working with attribute_groups provided by <linux/sysfs.h>
work with const attribute_group. These attribute_group structures do not
change at runtime so mark them as const.

File size before:
 text      data     bss     dec     hex filename
 2033      1448       0    3481     d99 drivers/atm/adummy.o

File size after:
 text      data     bss     dec     hex filename
 2129      1352       0    3481     d99 drivers/atm/adummy.o

This change was made with the help of Coccinelle.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>