ath6kl: 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 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/20200504200838.GA31974@embeddedor

drivers/net/wireless/ath/ath6kl/core.h

@@ -160,7 +160,7 @@ enum ath6kl_fw_capability {
 struct ath6kl_fw_ie {
 	__le32 id;
 	__le32 len;
-	u8 data[0];
+	u8 data[];
 };
 
 enum ath6kl_hw_flags {
@@ -406,7 +406,7 @@ struct ath6kl_mgmt_buff {
 	u32 id;
 	bool no_cck;
 	size_t len;
-	u8 buf[0];
+	u8 buf[];
 };
 
 struct ath6kl_sta {

drivers/net/wireless/ath/ath6kl/debug.c

@@ -30,7 +30,7 @@ struct ath6kl_fwlog_slot {
 	__le32 length;
 
 	/* max ATH6KL_FWLOG_PAYLOAD_SIZE bytes */
-	u8 payload[0];
+	u8 payload[];
 };
 
 #define ATH6KL_FWLOG_MAX_ENTRIES 20

drivers/net/wireless/ath/ath6kl/hif.h

@@ -199,7 +199,7 @@ struct hif_scatter_req {
 
 	u32 scat_q_depth;
 
-	struct hif_scatter_item scat_list[0];
+	struct hif_scatter_item scat_list[];
 };
 
 struct ath6kl_irq_proc_registers {