wpa: Address CTRL-EVENT-SCAN-FAILED

Some installations may experience CTRL-EVENT-SCAN-FAILED when
associating to an AP. Installations that specify
ifconfig_wlan0="WPA ... up" in rc.conf do not experience
the problem whereas those which specify ifconfig_wlan0="WPA" without
the "up" will experience CTRL-EVENT-SCAN_FAILED.

However those that specify "up" in ifconfig_wlan0 will be able to
reproduce this problem by service netif stop wlan0;
service netif start wlan0. Interestingly The service netif stop/start
problem is reproducible on the older wpa 2.9 as well.

Reported by: dhw
Reported by: "Oleg V. Nauman" <oleg _ theweb_org_ua>
Reported by: Filipe da Silva Santos <contact _ shiori_com_br>
Reported by: Jakob Alvermark <jakob _ alvermark_net>
MFC after: 3 days