WAN: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/wan/farsync.h

···
 struct fstioc_write {
         unsigned int  size;
         unsigned int  offset;
-        unsigned char data[0];
+	unsigned char data[];
 };
 
 

drivers/net/wan/wanxl.c

···
 	struct sk_buff *rx_skbs[RX_QUEUE_LENGTH];
 	struct card_status *status;	/* shared between host and card */
 	dma_addr_t status_address;
-	struct port ports[0];	/* 1 - 4 port structures follow */
+	struct port ports[];	/* 1 - 4 port structures follow */
 };