Bluetooth: Don't let background scanning interfering with discovery

If we have an active discovery going on we shouldn't do any changes to
LE scanning when hci_update_background_scan() is called (a call which
can happen for many different reasons). This patch fixes the issue by
returning from the function if the discovery state is anything else
except DISCOVERY_STOPPED.

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