Bluetooth: Add extra discovery fields for storing filter information

With the upcoming addition of support for Start Service Discovery, the
discovery handling needs to filter on RSSI and UUID values. For that
they need to be stored in the discovery handling. This patch adds the
appropiate fields and also make sure they are reset when discovery
has been stopped.

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