staging: comedi: ni_660x: add device-global routing

Provides the device-global routing interface for ni_660x devices.  Using
the device-global names in comedi_cmd structures for commands was already
supported through the ni_tio module.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>