Bluetooth: mgmt: multi adv for enable_advertising()

Previously enable_advertising() would rely on
get_adv_instance_scan_rsp_len() which checked for a hard coded instance
identifier. This is being changed to check for the current advertising
instance's scan response length instead. The function is renamed
accordingly.

Signed-off-by: Florian Grandel <fgrandel@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>