USB: atm: 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 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200211232148.GA20644@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/atm/ueagle-atm.c

@@ -350,7 +350,7 @@ struct l1_code {
 	u8 string_header[E4_L1_STRING_HEADER];
 	u8 page_number_to_block_index[E4_MAX_PAGE_NUMBER];
 	struct block_index page_header[E4_NO_SWAPPAGE_HEADERS];
-	u8 code[0];
+	u8 code[];
 } __packed;
 
 /* structures describing a block within a DSP page */