Bluetooth: Fix setting correct flags in AD

A recent change added MGMT_ADV_FLAG_DISCOV to the flags returned by
get_adv_instance_flags(), however failed to take into account limited
discoverable mode. This patch fixes the issue by setting the correct
discoverability flag in the AD data.

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