settings: Fix default value of flash area ID in Kconfig

Use the value extracted from DT instead of a hardcoded 4, as it is done
in commit 977b292d8072e4024664cf7293d42e532de64cd4 for tests.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
1 file changed