Bluetooth: Fix reporting invalid RSSI for LE devices

Start Discovery was reporting 0 RSSI for invalid RSSI only for
BR/EDR devices. LE devices were reported with RSSI 127.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org # 3.19+