zd1211rw: 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>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200305111216.GA24982@embeddedor

drivers/net/wireless/zydas/zd1211rw/zd_usb.h

@@ -69,7 +69,7 @@ enum control_requests {
 
 struct usb_req_read_regs {
 	__le16 id;
-	__le16 addr[0];
+	__le16 addr[];
 } __packed;
 
 struct reg_data {
@@ -79,7 +79,7 @@ struct reg_data {
 
 struct usb_req_write_regs {
 	__le16 id;
-	struct reg_data reg_writes[0];
+	struct reg_data reg_writes[];
 } __packed;
 
 enum {
@@ -95,7 +95,7 @@ struct usb_req_rfwrite {
 	/* 2: other (default) */
 	__le16 bits;
 	/* RF2595: 24 */
-	__le16 bit_values[0];
+	__le16 bit_values[];
 	/* (ZD_CR203 & ~(RF_IF_LE | RF_CLK | RF_DATA)) | (bit ? RF_DATA : 0) */
 } __packed;
 
@@ -118,7 +118,7 @@ struct usb_int_header {
 
 struct usb_int_regs {
 	struct usb_int_header hdr;
-	struct reg_data regs[0];
+	struct reg_data regs[];
 } __packed;
 
 struct usb_int_retry_fail {