wifi: carl9170: Replace fake flex-array with flexible-array member

Zero-length arrays as fake flexible 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:
drivers/net/wireless/ath/carl9170/tx.c:702:61: warning: array subscript i is outside array bounds of ‘const struct _carl9170_tx_status[0]’ [-Warray-bounds=]
drivers/net/wireless/ath/carl9170/tx.c:701:65: warning: array subscript i is outside array bounds of ‘const struct _carl9170_tx_status[0]’ [-Warray-bounds=]

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/267
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/ZBSl2M+aGIO1fnuG@work

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

@@ -320,9 +320,9 @@ struct carl9170_rsp {
 		struct carl9170_u32_list	rreg_res;
 		struct carl9170_u32_list	echo;
 #ifdef __CARL9170FW__
-		struct carl9170_tx_status	tx_status[0];
+		DECLARE_FLEX_ARRAY(struct carl9170_tx_status, tx_status);
 #endif /* __CARL9170FW__ */
-		struct _carl9170_tx_status	_tx_status[0];
+		DECLARE_FLEX_ARRAY(struct _carl9170_tx_status, _tx_status);
 		struct carl9170_gpio		gpio;
 		struct carl9170_tsf_rsp		tsf;
 		struct carl9170_psm		psm;