usb: gadget: f_phonet: 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 inadvertenly introduced[3] to the codebase from now on.

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")

---
 drivers/usb/gadget/function/f_phonet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 8b72b192c747..d7f6cc51b7ec 100644
@@ -48,7 +48,7 @@ struct f_phonet {
 	struct usb_ep			*in_ep, *out_ep;
 
 	struct usb_request		*in_req;
-	struct usb_request		*out_reqv[0];
+	struct usb_request		*out_reqv[];
 };
 
 static int phonet_rxq_size = 17;

2.25.0