mtd: Avoid printing error messages on probe deferrals

There is no reason to complain about probe errors in case of deferrals.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/linux-mtd/20230307192506.439532-1-miquel.raynal@bootlin.com