watchdog: s3c2410_wdt: Set timeout to actually achieved timeout

While rebasing my "watchdog_dev: Let the driver update the timeout field on
set_timeout success" patch (before I noticed it was already picked up by Wim),
I noticed that the s3c2410_wdt driver may not always have a 1 second
resolution, this patch changes s3c2410wdt_set_heartbeat to update the
timeout to the actually achieved timeout.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>