watchdog: mtk_wdt: Use MODE_KEY when stopping the watchdog

WDT_MODE value need to be or-ed with MODE_KEY when setting
watchdog mode. Add it to mtk_wdt_stop function, so that the
watchdog can be stopped (e.g. during suspend).

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>