staging: comedi: add NI USB-6501 support

Enable support for the National Instruments USB-6501 module.

The NI USB-6501 is a Full-Speed USB 2.0 (12 Mbit/s) device that
provides 24 digital I/O lines channels and one 32-bit counter.

This is a preliminary version:
	GPIO: works
	counter: doesn't work

Signed-off-by: Luca Ellero <luca.ellero@brickedbrain.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>