Bluetooth: hci_sync: hci_cmd_sync_queue_once() return -EEXIST if exists

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: hci_sync: hci_cmd_sync_queue_once() return -EEXIST if exists hci_cmd_sync_queue_once() needs to indicate whether a queue item was added, so caller can know if callbacks are called, so it can avoid leaking resources. Change the function to return -EEXIST if queue item already exists. Modify all callsites to handle that.