Bluetooth: Introduce helpers for LE set scan start and complete

Introduce a helper hci_req_start_scan() which starts an LE
scan and call it from passive_Scan() and active_scan().
There is not functionality change in this patch.

This is basically done to enable extended scanning if the
controller supports which will be done in the subsequent
patch

Signed-off-by: Jaganath Kanakkassery <jaganathx.kanakkassery@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>