prism54: 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 <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200507190210.GA15375@embeddedor

drivers/net/wireless/intersil/prism54/isl_oid.h

@@ -37,7 +37,7 @@ struct obj_mlmeex {
 	u16 state;
 	u16 code;
 	u16 size;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct obj_buffer {
@@ -68,12 +68,12 @@ struct obj_bss {
 
 struct obj_bsslist {
 	u32 nr;
-	struct obj_bss bsslist[0];
+	struct obj_bss bsslist[];
 } __packed;
 
 struct obj_frequencies {
 	u16 nr;
-	u16 mhz[0];
+	u16 mhz[];
 } __packed;
 
 struct obj_attachment {
@@ -81,7 +81,7 @@ struct obj_attachment {
 	char reserved;
 	short id;
 	short size;
-	char data[0];
+	char data[];
 } __packed;
 
 /*

drivers/net/wireless/intersil/prism54/islpci_mgt.h

@@ -99,7 +99,7 @@ struct islpci_mgmtframe {
 	pimfor_header_t *header;      /* payload header, points into buf */
 	void *data;		      /* payload ex header, points into buf */
         struct work_struct ws;	      /* argument for schedule_work() */
-	char buf[0];		      /* fragment buffer */
+	char buf[];		      /* fragment buffer */
 };
 
 int