Bluetooth: Controller: Fix CIS ISO Tx buffer leak on terminate

Fix CIS ISO Tx buffer leak on terminate when the datapath
has been deallocated.

Add a FIXME comment regarding ll_tx_ack_put() being called
from LLL execution context, while it is a ULL callable used
by ACL connections to dispatch ack.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>