Bluetooth: controller: legacy: Fix slave latency during conn update

Fix regression in cancelling slave latency during Connection
Update Procedure.

Slave latency should not be applied between the ack of a
Connection Update Indication PDU and until the instant.
When caching was introduced, implementation missed this
consideration.

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