Bluetooth: Fix mgmt_unpair_device command status

The default response status to unpair_device should be set as 0 instead
of a generic failure value. When disconnection is not needed (i.e. we
can reply imediately) we should return success and not failure.

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