Bluetooth: Controller: Fix TICKER_LOW_LAT Ext. Scan assert check

Fix Extended Scanning assertion check when using Ticker
Low Latency Implementation where the failure to schedule
a ticker timeout is returned inline compared to deferred
failure in the operation callback.

Auxiliary PDU is received using a single-shot ticker
timeout with ticks_slot reservation. This single-shot
ticker is allowed to fail if it is overlapping another
event.

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