greybus: make greybus_bus_type const

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

Cc: Alex Elder <elder@kernel.org>
Cc: greybus-dev@lists.linaro.org
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/2024010517-handgun-scoreless-05e7@gregkhSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/greybus/core.c

@@ -155,7 +155,7 @@ static void greybus_shutdown(struct device *dev)
 	}
 }
 
-struct bus_type greybus_bus_type = {
+const struct bus_type greybus_bus_type = {
 	.name =		"greybus",
 	.match =	greybus_match_device,
 	.uevent =	greybus_uevent,

include/linux/greybus.h

@@ -104,7 +104,7 @@ void gb_debugfs_init(void);
 void gb_debugfs_cleanup(void);
 struct dentry *gb_debugfs_get(void);
 
-extern struct bus_type greybus_bus_type;
+extern const struct bus_type greybus_bus_type;
 
 extern struct device_type greybus_hd_type;
 extern struct device_type greybus_module_type;