ffwizard: enable the ffuplink interface when sharenet is selected

The ffuplink interface is disabled by default on a fresh install.
During the course of running the wizard, the user must select if
they want to share their internet connection or not.  If the user
decides to share their internet, then the ffuplink interface shall
be enabled (setting disabled=0).

This addresses freifunk-berlin/firmware#603

Cherry-picked from d61ba66

utils/luci-app-ffwizard-berlin/luasrc/model/cbi/freifunk/assistent/wireless.lua

@@ -105,6 +105,11 @@ function main.write(self, section, value)
     --share internet was not enabled before, set to false now
     uci:set("ffwizard", "settings", "sharenet", "0")
     uci:save("ffwizard")
+    --in case the wizard has been re-ran, ensure ffuplink is disabled
+    uci:set("network", "ffuplink", "disabled", "1")
+  else
+    --sharenet was enabled, therefore enable the ffuplink network interface
+    uci:set("network", "ffuplink", "disabled", "0")
   end
 
   -- store wizard data to fill fields if wizard is rerun