Bluetooth: Enable duplicates filter in background scan

To avoid flooding the host with useless advertising reports during
background scan, we enable the duplicates filter from controller.

However, enabling duplicates filter requires a small change in
background scan routine in order to fix the following scenario:
  1) Background scan is running.
  2) A device disconnects and starts advertising.
  3) Before host gets the disconnect event, the advertising is reported
     to host. Since there is no pending LE connection at that time,
     nothing happens.
  4) Host gets the disconnection event and adds a pending connection.
  5) No advertising is reported (since controller is filtering) and the
     connection is never established.

So, to address this scenario, we should always restart background scan
to unsure we don't miss any advertising report (due to duplicates
filter).

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>