mtd: st_spi_fsm: 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/20220607152458.232847-3-u.kleine-koenig@pengutronix.de

drivers/mtd/devices/st_spi_fsm.c

@@ -2130,7 +2130,9 @@ static int stfsm_remove(struct platform_device *pdev)
 
 	clk_disable_unprepare(fsm->clk);
 
-	return mtd_device_unregister(&fsm->mtd);
+	WARN_ON(mtd_device_unregister(&fsm->mtd));
+
+	return 0;
 }
 
 #ifdef CONFIG_PM_SLEEP