Bluetooth: Controller: Make PDU octet3 vendor specific

Make Data, ISO, BIS and CIS PDU structure's octet 3 vendor
specific.

This will allow vendors not supporting the octet 3 or
CTEInfo (8 bits) to save 1 octet in their PDU structures.

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