Bluetooth: Controller: Store parent lll struct in aux context

We use 1st node enqueued in aux context to retrieve lll scan/sync
struct, but that only works if we buffer PDUs in aux context. It's
better to store parent lll struct as explicit member in aux context
as this also works if we skip buffering.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>