staging: comedi: add interface to ni routing table information

Adds interface and associated unittests for accessing/looking-up/validating
the new ni routing table information.

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>