mtd: physmap: Don't skip cleanup after mtd_device_unregister() failed

If mtd_device_unregister() fails (which it doesn't when used correctly),
the resources bound by the nand chip should be freed anyhow as returning
an error value doesn't prevent the device getting unbound.

Instead use WARN_ON on the return value similar to how other drivers do
it.

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/20220603210758.148493-14-u.kleine-koenig@pengutronix.de