brcmfmac: 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>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200225020804.GA9428@embeddedor

drivers/net/wireless/broadcom/brcm80211/brcmfmac/firmware.h

@@ -60,7 +60,7 @@ struct brcmf_fw_request {
 	u16 bus_nr;
 	u32 n_items;
 	const char *board_type;
-	struct brcmf_fw_item items[0];
+	struct brcmf_fw_item items[];
 };
 
 struct brcmf_fw_name {

drivers/net/wireless/broadcom/brcm80211/brcmfmac/fweh.c

@@ -32,7 +32,7 @@ struct brcmf_fweh_queue_item {
 	u8 ifaddr[ETH_ALEN];
 	struct brcmf_event_msg_be emsg;
 	u32 datalen;
-	u8 data[0];
+	u8 data[];
 };
 
 /**