mtd: rawnand: intel: check return value of devm_kasprintf()

devm_kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure. Ensure the allocation was successful by
checking the pointer validity.

Fixes: 0b1039f0 ("mtd: rawnand: Add NAND controller support on Intel LGM SoC")
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231019065537.318391-1-yiyang13@huawei.com