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.

fixes 84279c4a
11 jobs for wip/jonathankang/wifi-periodic-check in 4 minutes and 37 seconds (queued for 3 minutes and 21 seconds)
Status Job ID Name Coverage
  Build
passed #131411
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  Test
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asan

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