Bluetooth: mgmt: dry update_scan_rsp_data()

update_scan_rsp_data() duplicates code from get_current_adv_instance().
This is being fixed by letting the former make use of the latter.

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