board: st: common: fix dfu alt buffer clearing

board: st: common: 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: ec2933e543df ("board: stm32mp1: move set_dfu_alt_info in st common directory")
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>