mtd: rawnand: fsl_upm: Use devm_kasprintf() to allocate the MTD name

This simplifies the init() error path and the remove() handler.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200603134922.1352340-5-boris.brezillon@collabora.com