mtd: mchp48l640: 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 spi remove callback a generic error message by the
spi framework (and nothing else) is suppressed.
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/20211108093153.63792-3-u.kleine-koenig@pengutronix.de

drivers/mtd/devices/mchp48l640.c

@@ -345,7 +345,9 @@ static int mchp48l640_remove(struct spi_device *spi)
 {
 	struct mchp48l640_flash *flash = spi_get_drvdata(spi);
 
-	return mtd_device_unregister(&flash->mtd);
+	WARN_ON(mtd_device_unregister(&flash->mtd));
+
+	return 0;
 }
 
 static const struct of_device_id mchp48l640_of_table[] = {