Bluetooth: controller: fix node_rx retention mechanism

Ensure that in LLCP reference to node_rx is cleared when
retention is NOT used, to avoid corruption of node_rx later
re-allocated

Signed-off-by: Erik Brockhoff <erbr@oticon.com>
(cherry picked from commit 806a4fcf)
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>