Bluetooth: Controller: Fix ticker worker check ticks_slot_previous

Fix ticker worker from checking the ticks_slot_id_previous,
only ticks_slot_previous be checked as previous slot id
would be assigned to TICKER_NULL if the ticker was stopped.
The ticks_slot_previous value should be used to check that
a slot is reserved even if the ticker has been stopped.

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