Bluetooth: controller: Fix populate offset in latest advertising PDU

Fix implementation to populate the aux, and sync offset
in the latest PDU. If both the current and latest of the
double buffer has been filled and LLL did not pick the
latest PDU, then the offset should be filled into the latest
PDU (and not into the first/current PDU).

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