mtd: rawnand: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200226222722.GA18020@embeddedor

drivers/mtd/nand/raw/denali.h

@@ -328,7 +328,7 @@ struct denali_chip {
 	struct nand_chip chip;
 	struct list_head node;
 	unsigned int nsels;
-	struct denali_chip_sel sels[0];
+	struct denali_chip_sel sels[];
 };
 
 /**

drivers/mtd/nand/raw/marvell_nand.c

@@ -334,7 +334,7 @@ struct marvell_nand_chip {
 	int addr_cyc;
 	int selected_die;
 	unsigned int nsels;
-	struct marvell_nand_chip_sel sels[0];
+	struct marvell_nand_chip_sel sels[];
 };
 
 static inline struct marvell_nand_chip *to_marvell_nand(struct nand_chip *chip)

drivers/mtd/nand/raw/meson_nand.c

@@ -118,7 +118,7 @@ struct meson_nfc_nand_chip {
 	u8 *data_buf;
 	__le64 *info_buf;
 	u32 nsels;
-	u8 sels[0];
+	u8 sels[];
 };
 
 struct meson_nand_ecc {

drivers/mtd/nand/raw/mtk_nand.c

@@ -131,7 +131,7 @@ struct mtk_nfc_nand_chip {
 	u32 spare_per_sector;
 
 	int nsels;
-	u8 sels[0];
+	u8 sels[];
 	/* nothing after this field */
 };
 

drivers/mtd/nand/raw/nand_hynix.c

@@ -26,7 +26,7 @@
 struct hynix_read_retry {
 	int nregs;
 	const u8 *regs;
-	u8 values[0];
+	u8 values[];
 };
 
 /**

drivers/mtd/nand/raw/sunxi_nand.c

@@ -195,7 +195,7 @@ struct sunxi_nand_chip {
 	u32 timing_cfg;
 	u32 timing_ctl;
 	int nsels;
-	struct sunxi_nand_chip_sel sels[0];
+	struct sunxi_nand_chip_sel sels[];
 };
 
 static inline struct sunxi_nand_chip *to_sunxi_nand(struct nand_chip *nand)