Bluetooth: controller: Initialize scan rsp only for legacy adv

When legacy set is configured we need to make sure scan response is
properly initialized with empty data. For extended sets we should not
touch scan response on configure since data for those sets have to be
set explicitly, e.g. we cannot enable scannable advertising set if no
data was set.

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