Bluetooth: HCI: Add proper tracking for enable status of adv instances

This adds a field to track if advertising instances are enabled or not
and only clear HCI_LE_ADV flag if there is no instance left advertising.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>