mtd: rawnand: rockchip: Use flexible-array member instead of zero-length array

Suppresses the following coccinelle warning:

drivers/mtd/nand/raw/rockchip-nand-controller.c:162:4-8: WARNING use flexible-array member instead
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210323131137.45552-1-zou_wei@huawei.com

drivers/mtd/nand/raw/rockchip-nand-controller.c

@@ -159,7 +159,7 @@ struct rk_nfc_nand_chip {
 	u32 timing;
 
 	u8 nsels;
-	u8 sels[0];
+	u8 sels[];
 	/* Nothing after this field. */
 };