mtd: powernv_flash: Warn about failure to unregister mtd device

mtd_device_unregister() shouldn't fail. Wail loudly if it does anyhow.

This matches how other drivers (e.g. nand/raw/nandsim.c) use
mtd_device_unregister().

By returning 0 in the platform remove callback a generic error message
by the device core is suppressed, nothing else changes.

This is a preparation for making platform remove callbacks return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220603210758.148493-4-u.kleine-koenig@pengutronix.de

Author: Uwe Kleine-König

drivers/mtd/devices/powernv_flash.c

@@ -270,7 +270,9 @@ static int powernv_flash_release(struct platform_device *pdev)
 	struct powernv_flash *data = dev_get_drvdata(&(pdev->dev));
 
 	/* All resources should be freed automatically */
-	return mtd_device_unregister(&(data->mtd));
+	WARN_ON(mtd_device_unregister(&data->mtd));
+
+	return 0;
 }
 
 static const struct of_device_id powernv_flash_match[] = {