Bluetooth: Controller: Set/Clear function to enqueue aux PDU in caller

Refactor the ull_adv_aux_hdr_set_clear function so that the
caller of the function will enqueue the auxiliary PDU when
enqueuing the primary PDU.

This change is required to support chaining of AD Data where
parent auxiliary PDU will be added with aux ptr fields when
chain PDUs are appended.

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