Bluetooth: Report invalid RSSI for service discovery and background scan

When using Start Service Discovery and when background scanning is used
to report devices, the RSSI is reported or the value 127 is provided in
case RSSI in unavailable.

For Start Discovery the value 0 is reported to keep backwards
compatibility with the existing users.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>