fwu: developerbox: fix dfu alt buffer clearing

The set_dfu_alt_info() function calls the ALLOC_CACHE_ALIGN_BUFFER()
macro to declare a `buf' variable pointer into an array allocated on the
stack. It then calls the memset() function to clear the useable portion
of the array using the idiomatic expression `sizeof(buf)'.

While this would indeed work fine for an array, in the present case we
end up clearing only the size of a pointer.
Fix this by specifying the explicit size `DFU_ALT_BUF_LEN' instead.

Fixes: 6b403ca4dc ("fwu: DeveloperBox: add support for FWU")
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Masahisa Kojima <kojima.masahisa@socionext.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Jassi Brar <jaswinder.singh@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Link: https://lore.kernel.org/r/20250407170529.893307-2-vincent.stehle@arm.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>

board/socionext/developerbox/fwu_plat.c

@@ -18,7 +18,7 @@ void set_dfu_alt_info(char *interface, char *devstr)
 	struct mtd_info *mtd;
 	int ret;
 
-	memset(buf, 0, sizeof(buf));
+	memset(buf, 0, DFU_ALT_BUF_LEN);
 
 	mtd_probe_devices();