thunderbolt: Constify static attribute_group structs

The only usage of these is to put their addresses in arrays of pointers
to const attribute_groups. Make them const to allow the compiler to put
them in read-only memory.
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/thunderbolt/domain.c

@@ -289,7 +289,7 @@ static umode_t domain_attr_is_visible(struct kobject *kobj,
 	return attr->mode;
 }
 
-static struct attribute_group domain_attr_group = {
+static const struct attribute_group domain_attr_group = {
 	.is_visible = domain_attr_is_visible,
 	.attrs = domain_attrs,
 };

drivers/thunderbolt/switch.c

@@ -1771,7 +1771,7 @@ static umode_t switch_attr_is_visible(struct kobject *kobj,
 	return sw->safe_mode ? 0 : attr->mode;
 }
 
-static struct attribute_group switch_group = {
+static const struct attribute_group switch_group = {
 	.is_visible = switch_attr_is_visible,
 	.attrs = switch_attrs,
 };

drivers/thunderbolt/xdomain.c

@@ -756,7 +756,7 @@ static struct attribute *tb_service_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group tb_service_attr_group = {
+static const struct attribute_group tb_service_attr_group = {
 	.attrs = tb_service_attrs,
 };
 
@@ -1239,7 +1239,7 @@ static struct attribute *xdomain_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group xdomain_attr_group = {
+static const struct attribute_group xdomain_attr_group = {
 	.attrs = xdomain_attrs,
 };