watchdog: wdt-uclass.c: use wdt_start() in wdt_expire_now()

watchdog: wdt-uclass.c: use wdt_start() in wdt_expire_now()

wdt_start() does the "no ->start? return -ENOSYS" check, don't
open-code that in wdt_expire_now().

Also, wdt_start() maintains some global (and later some per-device)
state, which would get out of sync with this direct method call - not
that it matters much here since the board is supposed to reset very
soon.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>