mtd: rawnand: atmel: Warn about failure to unregister mtd device

The Linux device core doesn't intend remove callbacks to fail. If an
error code is returned the device is removed anyhow. So wail loudly if
the atmel specific remove callback fails and return 0 anyhow to suppress
the generic (and little helpful) error message by the device core.

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/20220607062503.211345-1-u.kleine-koenig@pengutronix.de