xilinx: zynq: 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: c67fecd2125b ("ARM: zynq: Enable capsule update for qspi and mmc")
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Michal Simek <michal.simek@amd.com>
Cc: Tom Rini <trini@konsulko.com>
Acked-by: Padmarao Begari <padmarao.begari@amd.com>
Link: https://lore.kernel.org/r/20250407170529.893307-5-vincent.stehle@arm.com
diff --git a/board/xilinx/zynq/board.c b/board/xilinx/zynq/board.c
index 5efef61..04dee1b 100644
--- a/board/xilinx/zynq/board.c
+++ b/board/xilinx/zynq/board.c
@@ -175,7 +175,7 @@
 {
 	ALLOC_CACHE_ALIGN_BUFFER(char, buf, DFU_ALT_BUF_LEN);
 
-	memset(buf, 0, sizeof(buf));
+	memset(buf, 0, DFU_ALT_BUF_LEN);
 
 	switch ((zynq_slcr_get_boot_mode()) & ZYNQ_BM_MASK) {
 #if defined(CONFIG_SPL_FS_LOAD_PAYLOAD_NAME)