Bluetooth: Controller: Fix Extended Scan and Periodic Sync terminate

Fix Extended Scan disable and Periodic Synchronization
terminate to cover scenarios where chain PDU reception could
complete while thread context is waiting for the radio event
to be done.

Changes here ensure auxiliary context is not released twice
which is caught as an assertion in flush() when checking for
validity of aux->parent pointer.

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