WPS: Fix clearing of SetSelectedRegistrar with multiple interfaces

WPS: Fix clearing of SetSelectedRegistrar with multiple interfaces

The SetSelectedRegistrar timeout was registered for each registrar
instance, but the only context pointer (struct subscription *) was
shared with each registrar which resulted in the timeout getting
cancelled for some of the registrar instances before the selected
registrar (ER) information was cleared.

In addition, when an ER unsubscribed from receiving events, the
selected registrar information got cleared only from a single
registrar.

Fix these issues by registering a pointer to the registrar
instance in the timeout and by iterating over all UPnP interfaces
when removing a subscription.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>