staging: comedi: tests: ni_routes_test: Fix compilation error

staging: comedi: tests: ni_routes_test: Fix compilation error

The `ni_routes_test` module is not currently selectable using the
Kconfig files, but can be built by specifying `CONFIG_COMEDI_TESTS=m` on
the "make" command line.  It currently fails to compile due to an extra
parameter added to the `ni_assign_device_routes` function by
commit e3b7ce73c578 ("staging: comedi: ni_routes: Allow alternate board
name for routes").  Fix it by supplying the value `NULL` for the added
`alt_board_name` parameter (which specifies that there is no alternate
board name).

Fixes: e3b7ce73c578 ("staging: comedi: ni_routes: Allow alternate board name for routes")
Cc: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210407140142.447250-2-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>