Stop the DPP Controller instance, if one is started, when the hostapd
interface that was used to start that Controller is removed. This is
needed to remove the control pointers that point to the soon-to-be-freed
hostapd structures. This fixes an issue where a Controller operation
with multiple interfaces could have resulted in references to freed
memory if an interface is removed without explicitly stopping the DPP
Controller.
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>