USB: 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>
Link: https://lore.kernel.org/r/20200220132017.GA29262@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb.h

@@ -325,7 +325,7 @@ struct usb_interface_cache {
 
 	/* variable-length array of alternate settings for this interface,
 	 * stored in no particular order */
-	struct usb_host_interface altsetting[0];
+	struct usb_host_interface altsetting[];
 };
 #define	ref_to_usb_interface_cache(r) \
 		container_of(r, struct usb_interface_cache, ref)
@@ -1589,7 +1589,7 @@ struct urb {
 	int error_count;		/* (return) number of ISO errors */
 	void *context;			/* (in) context for completion */
 	usb_complete_t complete;	/* (in) completion routine */
-	struct usb_iso_packet_descriptor iso_frame_desc[0];
+	struct usb_iso_packet_descriptor iso_frame_desc[];
 					/* (in) ISO ONLY */
 };