x86: acpi: Fix madt lapic generation

An accumulated length was incorrectly added to current each pass
through the loop. On system with more than 2 cores this caused a
corrupt MADT to be generated.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>