Fix per-SSID scan (scan_ssid=1)

Fix per-SSID scan (scan_ssid=1)

Commit d3a9822542166e7adec16e24622486ba90359ef5 broke per-SSID scan
by using the ssid variable for internal loop and by doing so,
overriding the value that was needed below to figure out whether the
scan is for a specific SSID. Fix this by using a temporary variable
instead when looping over network finding which frequencies to scan for.