thunderbolt: make tb_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the tb_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Andreas Noever <andreas.noever@gmail.com>
Cc: Michael Jamet <michael.jamet@intel.com>
Cc: Yehezkel Bernat <YehezkelShB@gmail.com>
Cc:  <linux-usb@vger.kernel.org>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/2023121904-utopia-broadcast-06d1@gregkhSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/domain.c

@@ -307,7 +307,7 @@ static const struct attribute_group *domain_attr_groups[] = {
 	NULL,
 };
 
-struct bus_type tb_bus_type = {
+const struct bus_type tb_bus_type = {
 	.name = "thunderbolt",
 	.match = tb_service_match,
 	.probe = tb_service_probe,

include/linux/thunderbolt.h

@@ -86,7 +86,7 @@ struct tb {
 	unsigned long privdata[];
 };
 
-extern struct bus_type tb_bus_type;
+extern const struct bus_type tb_bus_type;
 extern struct device_type tb_service_type;
 extern struct device_type tb_xdomain_type;