USB: consolidate usb_unbind_interface and usb_driver_release_interface

This patch (as1230) consolidates code in usb_unbind_interface() and
usb_driver_release_interface().  In fact, it makes release_interface
call unbind_interface, thereby removing the need for duplicated code.

It works like this: If the interface has already been registered with
the driver core when a driver releases it, then the usual driver-core
mechanism will call unbind_interface.  If it hasn't been unregistered
then we will make the call ourselves.

As a nice bonus, drivers now don't have to worry about whether their
disconnect method will get called when they release an interface -- it
always will.  Previously it would be called only if the interface was
registered.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>