Bluetooth: Add helper to set adv data

We wish to handle advertising data separately from advertising
parameters in our new MGMT requests. This change adds a helper that
allows the advertising data and scan response to be updated for an
existing advertising instance.

Reviewed-by: Sonny Sasaka <sonnysasaka@chromium.org>
Signed-off-by: Daniel Winkler <danielwinkler@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>