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 <[e-mail]>
Cc: Michael Jamet <[e-mail]>
Cc: Yehezkel Bernat <[e-mail]>
Cc: <[e-mail]>
Acked-by: Mika Westerberg <[e-mail]>
Link: https://lore.kernel.org/r/2023121904-utopia-broadcast-06d1@gregkh
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/thunderbolt/domain.c

···
 	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

···
 	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;