Bluetooth: controller: split: Schedule auxiliary after primary PDU

Fix to avoid a crash due to missing aux_ptr in current
primary PDU constructed by LE Set Extended Advertising
Parameter Set, while aux_ptr was populated by LE Set
Extended Advertising Data Set in the latest double buffered
PDU.

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