Bluetooth: Controller: Fix extended advertising set remove

Fix extended advertising set remove and clear, to reset the
PDU double buffer to keep one initialized PDU. This is done
to prevent common extended payload format contents from
being overwritten and corrupted due to same primary PDU
buffer being used to remove AdvA if auxiliary PDU is
allocated.

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