Here are minimal usb_find_interface() patches for the core, usblp and
scanner.

Basic design is:
- device major (USB_MAJOR for now) and minor are stored in probe()
   function to struct usb_interface as kdev_t
- open() can use new core function usb_find_interface() to find matching
  device in drivers device list
- disconnect() will set kdev_t struct usb_interface to NODEV, so open
  wont open it anymore without new probe()

I tested these patches and they work for me. I will work on small patches
of other work in these drivers (like removal of lock_kernel/unlock_kernel
in usblp, fixing the disconnect problems in both drivers etc.). Those
patches would be very small too, but there will be quite many.