Bluetooth: Controller: Fix to release aux context stored in node rx

Fix to release aux context stored in node rx, and not in the
superior scan or sync context as the one in the superior scan
or sync context could be reset or have a different new aux
context when multiple advertising chain reception is used.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
(cherry picked from commit a8065926)