watchdog: stm32_iwdg: fix error message during driver probe

The commit 3ab1663af6c1 ("watchdog: stm32_iwdg: Add pretimeout support")
introduces the support for the pre-timeout interrupt.

The support for this interrupt is optional but the driver uses the
platform_get_irq() which produces an error message during the driver
probe if we don't have any `interrupts` property in the DT.

Use the platform_get_irq_optional() API to get rid of the error message
as this property is optional.

Fixes: 3ab1663af6c1 ("watchdog: stm32_iwdg: Add pretimeout support")
Signed-off-by: Clément Le Goffic <[e-mail]>
Reviewed-by: Marek Vasut <[e-mail]>
Reviewed-by: Guenter Roeck <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

drivers/watchdog/stm32_iwdg.c

···
 	if (!wdt->data->has_early_wakeup)
 		return 0;
 
-	irq = platform_get_irq(pdev, 0);
+	irq = platform_get_irq_optional(pdev, 0);
 	if (irq <= 0)
 		return 0;