wifi: brcmfmac: firmware: Annotate struct brcmf_fw_request with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct brcmf_fw_request.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Arend van Spriel <aspriel@gmail.com>
Cc: Franky Lin <franky.lin@broadcom.com>
Cc: Hante Meuleman <hante.meuleman@broadcom.com>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: Matthias Brugger <mbrugger@suse.com>
Cc: Hector Martin <marcan@marcan.st>
Cc: "Alvin Šipraga" <alsi@bang-olufsen.dk>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: linux-wireless@vger.kernel.org
Cc: brcm80211-dev-list.pdl@broadcom.com
Cc: SHA-cyfmac-dev-list@infineon.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230915200552.never.642-kees@kernel.org

drivers/net/wireless/broadcom/brcm80211/brcmfmac/firmware.h

@@ -69,7 +69,7 @@ struct brcmf_fw_request {
 	u16 bus_nr;
 	u32 n_items;
 	const char *board_types[BRCMF_FW_MAX_BOARD_TYPES];
-	struct brcmf_fw_item items[];
+	struct brcmf_fw_item items[] __counted_by(n_items);
 };
 
 struct brcmf_fw_name {