- Remove struct intf_data. 
  It was silly, and gave us an unbalanced interface. 
  Instead, when removing a device_interface, all of a class's devices are
  iterated over, and intf->remove_device() is called for each. (Instead 
  of the device's list of interfaces it belongs to). 
  Interfaces are expected to tell if they support the device being removed.

- Remove struct device::intf_list.

- Protect classes' device list access with devclass_sem, which is taken 
  when a device or interface is added or removed. This allows access to the
  list w/o taking the class's lock, which allows subordinate objects to be
  registered (e.g. the interface itself, or objects the interfaces create
  for a device). 

- This should fix a deadlock when devices are added to interfaces, which 
  calls intf->add_device(), then interface_add_data(), which call 
  kobject_register() and hangs, since it tries to take the class's lock
  again.