Bluetooth: controller: Add non-connectable scannable

This enables processing of AUX_SCAN_REQ and sending back AUX_SCAN_RSP
on non-connectabe scannable advertising instances. Similar path will
be used for AUX_CONNECT_REQ so there are few references for connectable
as well, but this is not supported as for now.

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