watchdog: renesas_wdt: don't round closest with get_timeleft

We should never return more time left than there actually is. So, switch
to a plain divider instead of DIV_ROUND_CLOSEST.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>