Bluetooth: Controller: Ensure ext header max size reservation

Ensure maximum size of common extended header format be
reserved when CONFIG_BT_CTLR_ADV_RESERVE_MAX=n is used so
that changes to ACAD, like channel map update does not need
frequent update to periodic advertising auxiliary channel
PDU time reservations.

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