ade: Fix usage of undefined variable

`flake8` did report a typo:

    tools/ade:217:23: F821 undefined name 'user'

As far as I can tell nothing ever tries to stringify the `Device`
object, so that's probably dead code and that's why we never noticed.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>