iwlegacy: Replace zero-length arrays with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/223
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.htmlSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/YzIvzc0jsYLigO8a@work

drivers/net/wireless/intel/iwlegacy/commands.h

@@ -1710,7 +1710,7 @@ struct il4965_tx_resp {
 	 */
 	union {
 		__le32 status;
-		struct agg_tx_status agg_status[0];	/* for each agg frame */
+		DECLARE_FLEX_ARRAY(struct agg_tx_status, agg_status);	/* for each agg frame */
 	} u;
 } __packed;
 
@@ -3365,7 +3365,7 @@ struct il_rx_pkt {
 		struct il_compressed_ba_resp compressed_ba;
 		struct il_missed_beacon_notif missed_beacon;
 		__le32 status;
-		u8 raw[0];
+		DECLARE_FLEX_ARRAY(u8, raw);
 	} u;
 } __packed;