workqueue: make wq_subsys const

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

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Cc: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20240206-bus_cleanup-workqueue-v1-1-72b10d282d58@marliere.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>