staging: comedi: ni_atmio: make device_ids const

The `device_ids[]` passed to `MODULE_DEVICE_TABLE()` should be `const`.
When the "ni_atmio" driver is built-in, gcc warns about `device_ids`
being defined but ununsed.  Make it `const`.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>