wireless: realtek: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200225002746.GA26789@embeddedor

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu.h

@@ -627,7 +627,7 @@ struct rtl8xxxu_firmware_header {
 	u32	reserved4;
 	u32	reserved5;
 
-	u8	data[0];
+	u8	data[];
 };
 
 /*

drivers/net/wireless/realtek/rtlwifi/wifi.h

@@ -1051,13 +1051,13 @@ struct rtl_hdr_3addr {
 	u8 addr2[ETH_ALEN];
 	u8 addr3[ETH_ALEN];
 	__le16 seq_ctl;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 struct rtl_info_element {
 	u8 id;
 	u8 len;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct rtl_probe_rsp {
@@ -1068,7 +1068,7 @@ struct rtl_probe_rsp {
 	/*SSID, supported rates, FH params, DS params,
 	 * CF params, IBSS params, TIM (if beacon), RSN
 	 */
-	struct rtl_info_element info_element[0];
+	struct rtl_info_element info_element[];
 } __packed;
 
 /*LED related.*/

drivers/net/wireless/realtek/rtw88/fw.h

@@ -36,7 +36,7 @@ enum rtw_c2h_cmd_id_ext {
 struct rtw_c2h_cmd {
 	u8 id;
 	u8 seq;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 enum rtw_rsvd_packet_type {

drivers/net/wireless/realtek/rtw88/main.h

@@ -1640,7 +1640,7 @@ struct rtw_dev {
 	struct rtw_wow_param wow;
 
 	/* hci related data, must be last */
-	u8 priv[0] __aligned(sizeof(void *));
+	u8 priv[] __aligned(sizeof(void *));
 };
 
 #include "hci.h"