tests: Make dbus_old_wps_pbc more robust

Flush the cfg80211 scan cache explicitly to avoid false failure reports
if a BSS entry from an earlier test case remain. Such a failure could be
hit, e.g., with the following test case sequence:
wpas_mesh_mode_scan p2p_channel_random_social dbus_old_wps_pbc

Signed-off-by: Jouni Malinen <j@w1.fi>

tests/hwsim/test_dbus_old.py

@@ -637,6 +637,7 @@ def test_dbus_old_wps_pbc(dev, apdev):
 def _test_dbus_old_wps_pbc(dev, apdev):
     (bus,wpas_obj,path,if_obj) = prepare_dbus(dev[0])
 
+    dev[0].flush_scan_cache()
     hapd = start_ap(apdev[0])
     hapd.request("WPS_PBC")
     bssid = apdev[0]['bssid']