ARM: ixp4xx: Replace 0-length arrays with flexible arrays

Zero-length arrays are deprecated[1]. Replace npe_load_firmware's
union of 0-length arrays with flexible arrays. Detected with GCC 13,
using -fstrict-flex-arrays=3:

drivers/soc/ixp4xx/ixp4xx-npe.c: In function 'npe_load_firmware':
drivers/soc/ixp4xx/ixp4xx-npe.c:570:60: warning: array subscript i is outside array bounds of 'u32[0]' {aka 'unsigned int[]'} [-Warray-bounds=]
  570 |                         image->data[i] = swab32(image->data[i]);
include/uapi/linux/swab.h:115:54: note: in definition of macro '__swab32'
  115 | #define __swab32(x) (__u32)__builtin_bswap32((__u32)(x))
      |                                                      ^
drivers/soc/ixp4xx/ixp4xx-npe.c:570:42: note: in expansion of macro 'swab32'
  570 |                         image->data[i] = swab32(image->data[i]);
      |                                          ^~~~~~
drivers/soc/ixp4xx/ixp4xx-npe.c:522:29: note: while referencing 'data'
  522 |                         u32 data[0];
      |                             ^~~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: Krzysztof Halasa <khalasa@piap.pl>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230105215706.never.027-kees@kernel.org

drivers/soc/ixp4xx/ixp4xx-npe.c

@@ -519,15 +519,15 @@ int npe_load_firmware(struct npe *npe, const char *name, struct device *dev)
 		u32 id;
 		u32 size;
 		union {
-			u32 data[0];
-			struct dl_block blocks[0];
+			DECLARE_FLEX_ARRAY(u32, data);
+			DECLARE_FLEX_ARRAY(struct dl_block, blocks);
 		};
 	} *image;
 
 	struct dl_codeblock {
 		u32 npe_addr;
 		u32 size;
-		u32 data[0];
+		u32 data[];
 	} *cb;
 
 	int i, j, err, data_size, instr_size, blocks, table_end;