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>

drivers/usb/serial/io_usbvend.h

@@ -593,7 +593,7 @@ struct ti_i2c_desc {
 	__u8	Type;			// Type of descriptor
 	__le16	Size;			// Size of data only not including header
 	__u8	CheckSum;		// Checksum (8 bit sum of data only)
-	__u8	Data[0];		// Data starts here
+	__u8	Data[];		// Data starts here
 } __attribute__((packed));
 
 // for 5152 devices only (type 2 record)
@@ -601,7 +601,7 @@ struct ti_i2c_desc {
 struct ti_i2c_firmware_rec {
 	__u8	Ver_Major;		// Firmware Major version number
 	__u8	Ver_Minor;		// Firmware Minor version number
-	__u8	Data[0];		// Download starts here
+	__u8	Data[];		// Download starts here
 } __attribute__((packed));