sim: Fix flash_area_get_device_id in sim

commit c304a7f8
(refs/bisect/good-c304a7f8)
Author: Dominik Ermel <dominik.ermel@nordicsemi.no>
Date:   Mon Jun 14 16:08:42 2021 +0000

    sim: Add flash_area_get_device_id implementation

Adds an implementation for this function in the simulator that just
returns 0.  Fix this to return the proper ID so that when tests start
checking for this, it will return the right value.

Signed-off-by: David Brown <david.brown@linaro.org>