luci-mod-network: properly reflect config state in sysfs tristates

When a boolean sysfs option is forcibly set to `0` or `1` in uci, then make
sure to properly reflect that choice state when rendering the widget.

Right now the dropdown incorrectly reverted to "automatic" after saving but
not applying the changes.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>