power: reset: at91-poweroff: do not procede if at91_shdwc is allocated

There should be only one instance of struct shdwc in the system. This is
referenced through at91_shdwc. Return in probe if at91_shdwc is already
allocated.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/reset/at91-sama5d2_shdwc.c

@@ -255,6 +255,9 @@ static int __init at91_shdwc_probe(struct platform_device *pdev)
 	if (!pdev->dev.of_node)
 		return -ENODEV;
 
+	if (at91_shdwc)
+		return -EBUSY;
+
 	at91_shdwc = devm_kzalloc(&pdev->dev, sizeof(*at91_shdwc), GFP_KERNEL);
 	if (!at91_shdwc)
 		return -ENOMEM;