Don't start second scan when changing scan interval

Don't start second scan when changing scan interval

If a scan is currently running and the scan interval is changed, a
second scan will be started before the current has finished. This will
in turn, if no networks are configured, cause wpa_s->state to be
forced to WPA_INACTIVE before the first scan has finished.

Signed-hostap: Pontus Fuchs <pontus.fuchs@gmail.com>