subsys/settings: fcb compress areas only once

fcb instance might use less areas than CONFIG_SETTINGS_FCB_NUM_AREAS.
During compression of completely filled up fcb it was possible to try
compress areas more than once.

What this patch fixes is not a bug - rather fix for inefficient service
of the corner case.

fixes #12657

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>