wpa: Address CTRL-EVENT-SCAN-FAILED

5fcdc19a81 didn't fully resolve the issue. There remains a report
that an ifconfig wlan0 up by itself is insufficient. Ifconfig down
must precede it.

Reported by:	Filipe da Silva Santos <contact _ shiori_com_br>
Fixes:		5fcdc19a81
MFC after:	3 days

libexec/rc/rc.d/wpa_supplicant

@@ -29,6 +29,8 @@ is_ndis_interface()
 }
 
 wpa_poststart() {
+	ifconfig ${ifn} down
+	sleep 2
 	ifconfig ${ifn} up
 }