Lock interactive auth as busy until state change

This keeps interactive auth locked as busy until a state change so that e.g.
accepting a terms step will remain "busy looking" until the next step of the
process appears.

Fixes https://github.com/vector-im/riot-web/issues/12546

src/components/structures/InteractiveAuth.js

@@ -161,6 +161,7 @@ export default createReactClass({
     _authStateUpdated: function(stageType, stageState) {
         const oldStage = this.state.authStage;
         this.setState({
+            busy: false,
             authStage: stageType,
             stageState: stageState,
             errorText: stageState.error,
@@ -184,10 +185,6 @@ export default createReactClass({
                 errorText: null,
                 stageErrorText: null,
             });
-        } else {
-            this.setState({
-                busy: false,
-            });
         }
     },