Bluetooth: controller: Add association between adv, sync and iso

Add association between extended advertising, periodic
advertising and broadcast ISO instance, so that attempts to
create and terminate BIG can detect error conditions.

Error conditions being, trying to create BIG without a valid
periodic advertising train, or terminating BIG without prior
creation.

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