WPS: Change concurrent radio AP to use only one WPS UPnP instance

WPS: Change concurrent radio AP to use only one WPS UPnP instance

WPS external Registrars can get confused about multiple UPnP
instances (one per radio) on a dual-concurrent APs. Simplify the
design by sharing a single UPnP state machine for all wireless
interfaces controlled by hostapd. This matches with the previous
changes that made a single command enable WPS functionality on
all interfaces.

This is relatively minimal change to address the sharing of the
state among multiple struct hostapd_data instances. More cleanup
can be done separately to remove unnecessary copies of information.