firewire: Replace zero-length array with flexible-array member

Zero-length and one-element arrays are deprecated, and we are moving
towards adopting C99 flexible-array members, instead.

Address the following warnings found with GCC-13 and
-fstrict-flex-arrays=3 enabled:
sound/firewire/amdtp-stream.c: In function ‘build_it_pkt_header’:
sound/firewire/amdtp-stream.c:694:17: warning: ‘generate_cip_header’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
  694 |                 generate_cip_header(s, cip_header, data_block_counter, syt);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/firewire/amdtp-stream.c:694:17: note: referencing argument 2 of type ‘__be32[2]’ {aka ‘unsigned int[2]’}
sound/firewire/amdtp-stream.c:667:13: note: in a call to function ‘generate_cip_header’
  667 | static void generate_cip_header(struct amdtp_stream *s, __be32 cip_header[2],
      |             ^~~~~~~~~~~~~~~~~~~

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/303
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZHT0V3SpvHyxCv5W@work
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

include/linux/firewire.h

@@ -391,7 +391,7 @@ struct fw_iso_packet {
 	u32 tag:2;		/* tx: Tag in packet header		*/
 	u32 sy:4;		/* tx: Sy in packet header		*/
 	u32 header_length:8;	/* Length of immediate header		*/
-	u32 header[0];		/* tx: Top of 1394 isoch. data_block	*/
+	u32 header[];		/* tx: Top of 1394 isoch. data_block	*/
 };
 
 #define FW_ISO_CONTEXT_TRANSMIT			0