Bluetooth: Controller: Fix scan aux leak on periodic sync terminate

Fix scan aux context leak on Periodic Sync Terminate. On
Periodic Sync Terminate the `is_stop` flag is set to prevent
the ULL from assigning an aux context to the sync context,
but the acquired aux context was neither assigned to sync
context nor was being released back to the aux context pool.
Fixed by releasing the aux context if not assigned to the
sync context when `is_stop` flag is set.

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