Bluetooth: Controller: Fix aux scheduling from LLL

We need to alloc node prior to calling lll_scan_aux_setup, otherwise
the same node will be peeked inside that function and it will overwrite
some data in our node (e.g. "extra" field).

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>