Bluetooth: Fix discovery state machine

In case of Start Discovery command failure, we should set the discovery
state to DISCOVERY_STOPPED. Otherwise, we stuck at DISCOVERY_STARTING
state and subsequent Start Discovery commands will simply fail.

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>