Bluetooth: hci_sync: hold hdev->lock for hci_conn_params lookups

hci_conn_params_lookup requires hdev->lock be held, otherwise the list
iteration or param access is not safe.

Hold hdev->lock for params lookups in hci_sync.

Fixes: c530569adc19 ("Bluetooth: hci_core: Introduce HCI_CONN_FLAG_PAST")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>