wifi: ray_cs: Replace 1-element array with flexible array

The trailing array member of struct tx_buf was defined as a 1-element
array, but used as a flexible array. This was resulting in build warnings:

    In function 'fortify_memset_chk',
        inlined from 'memset_io' at /kisskb/src/arch/mips/include/asm/io.h:486:2,
        inlined from 'build_auth_frame' at /kisskb/src/drivers/net/wireless/legacy/ray_cs.c:2697:2:
    /kisskb/src/include/linux/fortify-string.h:493:25: error: call to '__write_overflow_field' declared with attribute warning:
detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
      493 |                         __write_overflow_field(p_size_field, size);
          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Replace it with an actual flexible array. Binary difference comparison
shows a single change in output:

│  drivers/net/wireless/legacy/ray_cs.c:883
│       lea    0x1c(%rbp),%r13d
│ -     cmp    $0x7c3,%r13d
│ +     cmp    $0x7c4,%r13d

This is from:

        if (len + TX_HEADER_LENGTH > TX_BUF_SIZE) {

specifically:

 #define TX_BUF_SIZE (2048 - sizeof(struct tx_msg))

This appears to have been originally buggy, so the change is correct.
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/all/88f83d73-781d-bdc-126-aa629cb368c@linux-m68k.org
Cc: Kalle Valo <kvalo@kernel.org>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230728231245.never.309-kees@kernel.org

drivers/net/wireless/legacy/rayctl.h

@@ -577,7 +577,7 @@ struct tx_msg {
     struct tib_structure tib;
     struct phy_header phy;
     struct mac_header mac;
-    UCHAR  var[1];
+    UCHAR  var[];
 };
 
 /****** ECF Receive Control Structure (RCS) Area at Shared RAM offset 0x0800  */