drivers: watchdog: wdt-uclass: Use IS_ENABLED for WATCHDOG_AUTOSTART

There is no separate SPL/TPL config for WATCHDOG_AUTOSTART.
So use IS_ENABLED instead of CONFIG_IS_ENABLED to make watchdog
working in SPL again.

Fixes: 830d29ac ("watchdog: Allow to use CONFIG_WDT without starting watchdog")
Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Stefan Roese <sr@denx.de>