driver_unregister — remove driver from system.
void driver_unregister (struct device_driver * drv);
Again, we pass off most of the work to the bus-level call.
Though, once that is done, we wait until drv->unloaded is completed.
This will block until the driver refcount reaches 0, and it is
released. Only modular drivers will call this function, and we
have to guarantee that it won't complete, letting the driver
unload until all references are gone.