Bluetooth: host: gatt: don't use settings API in disconnect cb

When `CONFIG_BT_SETTINGS_DELAYED_STORE` is enabled, reschedule the storage
work immediately instead of processing it in the current context.

When that config is not enabled, process the storage in the current
context.

Processing this work (i.e. using the settings API and its likely
FLASH-based backend) in the current context (cooperative prio) may mess
with the real-time constraints of the application threads.

Fixes #55067.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>