wifi: ath10k: 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/212
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 <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/YzIcoloIQBDqGlgc@work

drivers/net/wireless/ath/ath10k/htt.h

@@ -1108,8 +1108,10 @@ struct htt_rx_in_ord_ind {
 	u8 reserved;
 	__le16 msdu_count;
 	union {
-		struct htt_rx_in_ord_msdu_desc msdu_descs32[0];
-		struct htt_rx_in_ord_msdu_desc_ext msdu_descs64[0];
+		DECLARE_FLEX_ARRAY(struct htt_rx_in_ord_msdu_desc,
+				   msdu_descs32);
+		DECLARE_FLEX_ARRAY(struct htt_rx_in_ord_msdu_desc_ext,
+				   msdu_descs64);
 	} __packed;
 } __packed;