Bluetooth: Fix event sending with DISCOVERY_STOPPED state

We are not supposed to send mgmt_discovering events if we are transiting
from DISCOVERY_STARTING to DISCOVERY_STOPPED state. It doesn't make
sense to send mgmt_discovering event once discovery procedure has not
been even started.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>