Bluetooth: controller: Fix assertion on Ext. Advertising disable

Fix an assertion when disabling Extended Advertising with
auxiliary PDUs. Stop the auxiliary PDU scheduling after the
primary PDU scheduling is stopped first. This will avoid the
assertion caused due to a search for a stopped auxiliary PDU
offset by the primary PDU scheduling that is being stopped.

Fixes #32866.

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