irqchip/imx-intmux: Use struct_size() helper in devm_kzalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/20200616230923.GA24937@embeddedor

drivers/irqchip/irq-imx-intmux.c

···
 		return -EINVAL;
 	}
 
-	data = devm_kzalloc(&pdev->dev, sizeof(*data) +
-			    channum * sizeof(data->irqchip_data[0]), GFP_KERNEL);
+	data = devm_kzalloc(&pdev->dev, struct_size(data, irqchip_data, channum), GFP_KERNEL);
 	if (!data)
 		return -ENOMEM;