Bluetooth: hci_sync: hold hdev->lock when cleanup hci_conn

When disconnecting all devices, hci_conn_failed is used to cleanup
hci_conn object when the hci_conn object cannot be aborted.
The function hci_conn_failed requires the caller holds hdev->lock.

Fixes: 9b3628d7 ("Bluetooth: hci_sync: Cleanup hci_conn if it cannot be aborted")
Signed-off-by: Zhengping Jiang <jiangzp@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>