wpa_supplicant: reduce scan timeout to workaround ar6000

Ar6k hangs on the first scan after resume causing slow wifi reconnects
as wpa_supplicant must timeout (30s) and then retry. Work around the
binary module bug by setting the wpa_supplicant scan timeout to be much
shorter. Hopefully 5s is still long enough. Scans typically take 2-3s
on my network.