Staging: comedi: add comedi_usb_auto_[un]config functions

This will be used by the usbdux and usbduxfast drivers.

From: Bernd Porr <BerndPorr@f2s.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: David Schleef <ds@schleef.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>