Bluetooth: Fix discovery filter when no RSSI is available

When no RSSI value is available then make sure that the result is
filtered out when the RSSI threshold filter is active.

This means that all Bluetooth 1.1 or earlier devices will not
report any results when using a RSSI threshold filter.

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