wext: don't overwrite BSS frequency

wext: don't overwrite BSS frequency

mac80211 sends _both_ channel and frequency in it's scan results, with
frequency first and channel second (it's since been fixed to send
channel first and frequency second to work around this issue).  This
results in wpa_supplicant getting the right value when the frequency
comes, but overwriting the value with '0' when the channel comes because
wpa_supplicant can't handle 5GHz channel numbers.  So if a valid
previous SIOCGIWFREQ event came in, don't try to overwrite it.