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 <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/devices/powernv_flash.c

···
 	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[] = {