Bluetooth: Temporarily background scanning on Discovery

If the user send a mgmt start discovery command while the background
scanning is running, we should temporarily stop it. Once the discovery
finishes, we start the background scanning again.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Bluetooth: Fix hci_untrigger_background_scan()

If device discovery is running, we should not stop the scanning in
hci_untrigger_background_scan(). Otherwise, we brake the discovery
state machine.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>