hci_cmd_sync_queue can be called multiple times, each adding a
hci_cmd_sync_work_entry, before hci_cmd_sync_work is run so this makes
sure they are all dequeued properly otherwise it creates a backlog of
entries that are never run.

Link: https://lore.kernel.org/all/CAJCQCtSeUtHCgsHXLGrSTWKmyjaQDbDNpP4rb0i+RE+L2FTXSA@mail.gmail.com/T/
Fixes: 6a98e383 ("Bluetooth: Add helper for serialized HCI command execution")
Tested-by: NChris Clayton <chris2553@googlemail.com>
Signed-off-by: NLuiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>