device_unregister() is intended to be the complement of device_register(), and assumes
most of the functionality of the current put_device(). It should be called by the bus 
driver when a device is physically removed from the system. 

It should _not_ be called from a driver's remove() method, as that remove() method is called
via device_detach() in device_unregister(). 

dev->present is used to flag the physical presence of the device. It is set when the device
is registered, and cleared when unregistered. get_device() checks this flag, and returns
a NULL pointer if cleared. This prevents anyone from obtaining a reference to a device
that has been unregistered (removed), but not yet been freed (e.g. if someone else is 
holding a reference to it).

put_device() BUG()s if dev->present is set. A device should be unregistered before it is 
freed. This will catch people doing it in the wrong order.