tests: Clear scan cache at the end of ap_wps_per_station_psk

It was possible for the WPS PBC state to get cached through to the
following test cases and that would trigger false failures. Fix this by
explicitly clearing the scan cache at the end of ap_wps_per_station_psk.
This issue was triggered with the following test case sequence:
ap_wps_per_station_psk autogo_pbc

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

tests/hwsim/test_ap_wps.py

@@ -2061,6 +2061,7 @@ def test_ap_wps_per_station_psk(dev, apdev):
     except:
         pass
 
+    hapd = None
     try:
         with open(pskfile, "w") as f:
             f.write("# WPA PSKs\n")
@@ -2113,6 +2114,14 @@ def test_ap_wps_per_station_psk(dev, apdev):
             raise Exception("Same PSK recorded for sta0(enrollee) and sta0(reg)")
     finally:
         os.remove(pskfile)
+        if hapd:
+            dev[0].request("DISCONNECT")
+            dev[1].request("DISCONNECT")
+            dev[2].request("DISCONNECT")
+            hapd.disable()
+            dev[0].flush_scan_cache()
+            dev[1].flush_scan_cache()
+            dev[2].flush_scan_cache()
 
 def test_ap_wps_per_station_psk_failure(dev, apdev):
     """WPS PBC provisioning with per-station PSK (file not writable)"""