Bluetooth: controller: Use decoded AdvA/TargetA for conn ind helper

The same helper will be used for legacy and extended advertising so we
better pass AdvA and TargetA directly instead of complete advertising
PDU. This is because those fields are at different locations in legacy
and extended advertising PDUs so caller can figure them out.

Also, we can now check for directed advertising using TargetA since it
will be only supplied for such advertising (NULL otherwise).

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