watchdog: mtk_wdt: signedness bug in mtk_wdt_start()

"ret" should be signed for the error handling to work correctly.  This
doesn't matter much in real life since mtk_wdt_set_timeout() always
succeeds.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>