wifi: Disable periodic scan only when Wi-Fi is disabled
The periodic Wi-Fi scan is introduced in 84279c4a. It's designed to scan the Wi-Fi list every 15 seconds and disable the scan when Wi-Fi is disabled.
But currently the periodic check is disabled every time wireless_enabled_toggled() is called, which is wrong. Because it can mean Wi-Fi is enabled or disabled.
Fix that by disabling periodic scan only when Wi-Fi is disabled.