irqchip/exiu: Fix return value check in exiu_init()

In case of error, the function of_iomap() returns NULL pointer not
ERR_PTR().

Replace the IS_ERR() test of the return value with NULL test and return
a proper error code.

Fixes: 706cffc1b912 ("irqchip/exiu: Add support for Socionext Synquacer EXIU controller")
Signed-off-by: Wei Yongjun <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Jason Cooper <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

drivers/irqchip/irq-sni-exiu.c

···
 	}
 
 	data->base = of_iomap(node, 0);
-	if (IS_ERR(data->base)) {
-		err = PTR_ERR(data->base);
+	if (!data->base) {
+		err = -ENODEV;
 		goto out_free;
 	}