bluetooth: controller: Prevent unnecesary ticker "must expire"

When running a ticker node as "must expire", the node would invoke the
ticker callback even when programmed with latency. As "must expire" is
intended for scheduled events which are skipped due to collision, and as
such expected by LLL, purposefully skipped events should not generate
"must expire" callbacks.

Signed-off-by: Morten Priess <mtpr@oticon.com>