Bluetooth: Controller: Reschedule unreserved ticker with slot window

Update ticker implementation to reschedule unreserved ticker
that use ticks_slot_window.

This will avoid unreserved ticker from continuously
overlapping with reserved tickers. And the change is
required to reduce processing overheads before reserve
ticker timeout callback executes.

Unreserved tickers with ticks_slot_window will always yield
to any colliding reserved or unreserved tickers.

The implementation moves unreserved tickers (no ticks_slot
value, in this case continuous scan window) to end of a
colliding reserved/unreserved ticker so that reserved
tickers (like central role) do not have high overheads due
to unreserved ticker's prepare (scanner) be preempted
everytime before reserved ticker's prepare (central).

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