watchdog: wdt-uclass.c: neaten UCLASS_DRIVER definition

The addition of .pre_probe and .per_device_auto made this look
bad. Fix it.

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