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>