USB: total removal of multithreaded probing in usb

The whole approach is simply wrong.  Forking a thread means that
  - errors are ignored
  - locking is ignored

Doing this correctly would require major surgery for questionable benefit.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>