- remove device from driver's list on device_detach 
- set device's driver to NULL
- decrement reference count on driver on device_detach
- remove devices from driver's list in driver_detach
- use a write_lock instead of read_lock
- don't lock around initialization of device fields

- assume we have a bus in __remove_driver (we enforce this in driver_register)
- do put_bus last in __remove_driver
- lock bus around atomic_set in remove_driver and atomic_dec_and_test in put_driver
- remove from bus's list while we have it locked