Bluetooth: Controller: Fix assertion terminating Periodic Sync

Fix assertion due to changes introduced to support multiple
advertising chain reception.

Auxiliary context association with the Periodic Sync context
was not cleared due to which when terminating a Periodic
Synchronization triggered an assertion check, under race
conditions, detecting that the auxiliary context was already
released.

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