ipw2x00: Replace zero-length array with flexible-array

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200507185451.GA14603@embeddedor

drivers/net/wireless/intel/ipw2x00/ipw2200.h

@@ -448,7 +448,7 @@ struct tfd_command {
 	u8 index;
 	u8 length;
 	__le16 reserved;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 struct tfd_data {
@@ -675,7 +675,7 @@ struct ipw_rx_frame {
 	// is identical)
 	u8 rtscts_seen;		// 0x1 RTS seen ; 0x2 CTS seen
 	__le16 length;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct ipw_rx_header {
@@ -1002,7 +1002,7 @@ struct ipw_cmd {	 /* XXX */
    * Incoming parameters listed 1-st, followed by outcoming params.
    * nParams=(len+3)/4+status_len
    */
-	u32 param[0];
+	u32 param[];
 } __packed;
 
 #define STATUS_HCMD_ACTIVE      (1<<0)	/**< host command in progress */
@@ -1108,7 +1108,7 @@ struct ipw_fw_error {	 /* XXX */
 	u32 log_len;
 	struct ipw_error_elem *elem;
 	struct ipw_event *log;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 #ifdef CONFIG_IPW2200_PROMISCUOUS
@@ -1153,7 +1153,7 @@ struct ipw_rt_hdr {
 	s8 rt_dbmsignal;	/* signal in dbM, kluged to signed */
 	s8 rt_dbmnoise;
 	u8 rt_antenna;	/* antenna number */
-	u8 payload[0];  /* payload... */
+	u8 payload[];  /* payload... */
 } __packed;
 #endif