driver model: make classes and interfaces use kobject infrastructure.

driver model: make classes and interfaces use kobject infrastructure.

Like the other objects, this allows a decent bit of cleanup. Details include:

- use rwsem in subsytem, instead of one in struct device_class.

- use refcount in struct kobject, instead of one in struct device_class.

- kill class's present flag.

- kill class_list, since we can just use class_subsys's.

- make interfaces instances of their class's subsystem. This allows us to
kill struct device_class::intf_list, and struct device_interface::node.