wireless: ti: 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 7649773293 ("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/20200225003408.GA28675@embeddedor

drivers/net/wireless/ti/wl1251/cmd.h

@@ -90,7 +90,7 @@ struct wl1251_cmd_header {
 	u16 id;
 	u16 status;
 	/* payload */
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct  wl1251_command {
@@ -281,7 +281,7 @@ struct wl1251_cmd_packet_template {
 	struct wl1251_cmd_header header;
 
 	__le16 size;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 #define TIM_ELE_ID    5

drivers/net/wireless/ti/wl1251/wl12xx_80211.h

@@ -65,7 +65,7 @@ struct ieee80211_header {
 	u8 sa[ETH_ALEN];
 	u8 bssid[ETH_ALEN];
 	__le16 seq_ctl;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 struct wl12xx_ie_header {

drivers/net/wireless/ti/wlcore/acx.h

@@ -938,7 +938,7 @@ struct acx_rx_filter_cfg {
 	u8 action;
 
 	u8 num_fields;
-	u8 fields[0];
+	u8 fields[];
 } __packed;
 
 struct acx_roaming_stats {

drivers/net/wireless/ti/wlcore/boot.h

@@ -26,7 +26,7 @@ struct wl1271_static_data {
 	u8 fw_version[WL1271_FW_VERSION_MAX_LEN];
 	u32 hw_version;
 	u8 tx_power_table[WL1271_NO_SUBBANDS][WL1271_NO_POWER_LEVELS];
-	u8 priv[0];
+	u8 priv[];
 };
 
 /* number of times we try to read the INIT interrupt */

drivers/net/wireless/ti/wlcore/cmd.h

@@ -209,7 +209,7 @@ struct wl1271_cmd_header {
 	__le16 id;
 	__le16 status;
 	/* payload */
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 #define WL1271_CMD_MAX_PARAMS 572

drivers/net/wireless/ti/wlcore/conf.h

@@ -1150,7 +1150,7 @@ struct wlcore_conf {
 struct wlcore_conf_file {
 	struct wlcore_conf_header header;
 	struct wlcore_conf core;
-	u8 priv[0];
+	u8 priv[];
 } __packed;
 
 #endif

drivers/net/wireless/ti/wlcore/wl12xx_80211.h

@@ -66,7 +66,7 @@ struct ieee80211_header {
 	u8 sa[ETH_ALEN];
 	u8 bssid[ETH_ALEN];
 	__le16 seq_ctl;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 struct wl12xx_ie_header {