mtd: rawnand: arasan: stop using 0 as NULL pointer

Fix the following sparse warnings:

drivers/mtd/nand/raw/arasan-nand-controller.c:918:70: warning: Using plain integer as NULL pointer
drivers/mtd/nand/raw/arasan-nand-controller.c:918:73: warning: Using plain integer as NULL pointer

Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220725112108.686347-1-gongruiqi1@huawei.com