Fix caching of get_vfo to avoid caching error returns

https://github.com/Hamlib/Hamlib/issues/270

src/rig.c

@@ -1987,10 +1987,10 @@ int HAMLIB_API rig_get_vfo(RIG *rig, vfo_t *vfo)
     if (retcode == RIG_OK)
     {
         rig->state.current_vfo = *vfo;
-    }
-
         rig->state.cache.vfo = *vfo;
         cache_ms = elapsed_ms(&rig->state.cache.time_vfo, ELAPSED_SET);
+    }
+
     return retcode;
 }