HID: multitouch: 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: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-multitouch.c

@@ -341,7 +341,7 @@ static struct attribute *sysfs_attrs[] = {
 	NULL
 };
 
-static struct attribute_group mt_attribute_group = {
+static const struct attribute_group mt_attribute_group = {
 	.attrs = sysfs_attrs
 };