Bluetooth: Host: Fix wrong ID being stored

Fix an issue causing a wrong Bluetooth identity value to be stored. It
was happening because the `bt_dev.id_count` was incremented after the
settings being stored.

To fix this, `bt_dev.id_count` is now incremented right before the ID
creation and is decremented if the ID creation failed.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>
(cherry picked from commit 36367676)