Bluetooth: Controller: Fix parent PDU's aux offset in aux ptr field

Fix Extended Advertising and Periodic Advertising's parent
PDU's aux ptr field to contain correct aux offset value to
its chain PDU.

Uninitialized pointer reference to aux ptr in the parent PDU
was used before the pointer reference was returned by the
function adding the aux ptr fields in the parent PDU.

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