staging: comedi: tests: ni_route_tests: Declare functions static

The "ni_routes_test" module contains a bunch of functions with external
linkage that are not called externally.  Declare them `static`.

Cc: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210407181342.1117754-3-abbotti@mev.co.uk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>