Bluetooth: controller: Send TX PDU release with no DP to vendor function

If a race condition occurs between stopping CIS stream and tearing down
data path, releasing TX PDUs was not possible for vendor data path, as
the DP configuration is gone.

In that case, call a new vendor specific function for cleaning up and
returning PDUs to correct pool.

Signed-off-by: Morten Priess <mtpr@oticon.com>