Bluetooth: Controller: Fix node rx leak on extended scanning stop

If scan is being disabled, rx could already be enqueued
before coming to ull_scan_aux_rx_flush. Check if node rx is
not the last in the list of received PDUs then add it to the
list, else do not add it to the list to avoid duplicate
report generation, release and probable infinite loop
processing of the list. It is possible that node rx has not
be already added in case of last PDU in the chain, under
this case when scan is being stopped add it to the list
before auxiliary context is flushed.

Add the node rx to aux context list of node rx if not
already added when coming to ull_scan_aux_rx_flush. This is
handling a race condition wherein the last PDU in chain is
received and at the same time scan is being disabled.

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