carl9170: Replace zero-length array 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: KSPP/linux#193
Link: KSPP/linux#215
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Signed-off-by: Gustavo A. R. Silva <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Kalle Valo <[email protected]>
Link: https://lore.kernel.org/r/YzIdWc8QSdZFHBYg@work

Author: GustavoARSilva

drivers/net/wireless/ath/carl9170/wlan.h

@@ -271,7 +271,7 @@ struct ar9170_tx_frame {
 
 	union {
 		struct ieee80211_hdr i3e;
-		u8 payload[0];
+		DECLARE_FLEX_ARRAY(u8, payload);
 	} data;
 } __packed;