devm_kasprintf() can return a NULL pointer on failure but this
returned value is not checked.

Fixes: acfe63ec ("mtd: Convert to using %pOFn instead of device_node.name")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240828092427.128177-1-hanchunchao@inspur.com