This patch fixes a regression on setting up asynchronous commands to use
external trigger sources when board-specific routing information is
missing.

`ni_find_device_routes()` (called via `ni_assign_device_routes()`) finds
the table of register values for the device family and the set of valid
routes for the specific board.  If both are found,
`tables->route_values` is set to point to the table of register values
for the device family and `tables->valid_routes` is set to point to the
list of valid routes for the specific board.  If either is not found,
both `tables->route_values` and `tables->valid_routes` are left set at
their initial null values (initialized by `ni_assign_device_routes()`)
and the function returns `-ENODATA`.

Returning an error results in some routing functionality being disabled.
Unfortunately, leaving `table->route_values` set to `NULL` also breaks
the setting up of asynchronous commands that are configured to use
external trigger sources.  Calls to `ni_check_trigger_arg()` or
`ni_check_trigger_arg_roffs()` while checking the asynchronous command
set-up would result in a null pointer dereference if
`table->route_values` is `NULL`.  The null pointer dereference is fixed
in another patch, but it now results in failure to set up the
asynchronous command.  That is a regression from the behavior prior to
commit 347e2448 ("staging: comedi: tio: implement global tio/ctr
routing") and commit 56d0b826 ("staging: comedi: ni_mio_common:
implement new routing for TRIG_EXT").

Change `ni_find_device_routes()` to set `tables->route_values` and/or
`tables->valid_routes` to valid information even if the other one can
only be set to `NULL` due to missing information.  The function will
still return an error in that case.  This should result in
`tables->valid_routes` being valid for all currently supported device
families even if the board-specific routing information is missing.
That should be enough to fix the regression on setting up asynchronous
commands to use external triggers for boards with missing routing
information.

Fixes: 347e2448 ("staging: comedi: tio: implement global tio/ctr routing")
Fixes: 56d0b826 ("staging: comedi: ni_mio_common: implement new routing for TRIG_EXT").
Cc: <stable@vger.kernel.org> # 4.20+
Cc: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20200114182532.132058-3-abbotti@mev.co.ukSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>