bluetooth: gatt: Fix storing subscriptions

Change fixes storing subscriptions in settings. CCC write can interrupt
the ccc_delayed_store. Without the change, new CCC was not stored in
non-volatile memory and the work that stored CCC was not resubmitted.
That resulted in lost subscription after reboot.

Fixes: #26862

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>