arm64: zynqmp: fix dfu alt buffer clearing

arm64: zynqmp: 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: b86f43de0be0 ("xilinx: zynqmp: Add support for runtime dfu_alt_info setup")
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/20250407170529.893307-6-vincent.stehle@arm.com