firewall: fix save in translation mode closes #546 #547

This is part of #431, right fix is to use <button/> instead.  We
don't want to extend the code to fix this, it'll introduce more
bugs and strangeness later.  Still a lot to do there, any help is
welcome, the conversion is pretty simple.

src/www/firewall_nat_out.php

@@ -307,7 +307,7 @@ include("head.inc");
                   </tr>
                   <tr>
                     <td colspan="4">
-                      <input name="save" type="submit" class="btn btn-primary" value="<?=gettext("Save");?>" />
+                      <button name="save" type="submit" class="btn btn-primary" value="Save"><?= gettext('Save') ?></button>
                     </td>
                   </tr>
                 </tbody>