Bluetooth: Controller: Fix return value for unassigned aux context

Fix the return value use to indicate unassigned auxiliary
context when scanning is using LLL scheduling. This can
happen only under LLL scheduling where in LLL auxiliary
channel PDU reception has been spawn from LLL primary
channel scanning and on completion will join back to resume
primary channel PDU scanning.

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