mtd: rawnand: meson: 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: 1e4d3ba6 ("mtd: rawnand: meson: fix the clock")
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231019065548.318443-1-yiyang13@huawei.com