- make sure drv->devices is initialized on registration (from Peter Osterlund)
- add remove_driver for forcing removal of driver

There was a potential race with the module unload code. When a pci driver was unloaded, it would call pci_unregister_driver, which would simply call put_driver.
If the driver's refcount wasn't 0, it wouldn't unbind it from devices, but the module unload would still continue. 
If something tried to access the driver later (since everyone thinks its still there), Bad Things would happen. 
This fixes it until there can be tighter integration between the device model and module unload code.