gluon-luci-admin: fix firmware upgrade confirmation code

Fix an exception breaking the page.

package/gluon-luci-admin/files/usr/lib/lua/luci/view/admin/upgrade_confirm.htm

@@ -45,7 +45,7 @@ $Id$
       if flashsize > 0 then
         write(luci.i18n.translatef(
           " (%s available)",
-          w.byte_format(flashsize)
+          byte_format(flashsize)
         ))
       end
     %></li>