Bluetooth: buf: Put command complete/status in sync buf pool

Why is it ok to use the sync pool?

Because command complete/status is processed in prio: that means on the
same stack as the `bt_recv()` call from the driver.

Why does it fix the issue?

Because the complete/status event goes into a pool that is guaranteed to
have one free buffer any time `bt_recv()` is not executing.

Since the driver is the one calling bt_recv(), it (hopefully) will
finish one `bt_recv()` before starting another one.

Fixes #78223

Co-authored-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>
Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>
Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>