Do not continually reschedule specific scans to help finding hidden SSIDs

Do not continually reschedule specific scans to help finding hidden SSIDs

In situations where the driver does background scanning and sends a
steady stream of scan results, wpa_supplicant would continually
reschedule the scan.  This resulted in specific SSID scans never
happening for a hidden AP, and the supplicant never connecting to the AP
because it never got found.  Instead, if there's an already scheduled
scan, and a request comes in to reschedule it, and there are enabled
scan_ssid=1 network blocks, let the scan happen anyway so the hidden
SSID has a chance to be found.