acpi: simplify acpi_write_ssdt()

* Converting to void * is superfluous when calling memset().
* acpi_fill_header() already fills oem_table_id.

Fixes: d9531375 ("x86: Move SSDT table to a writer function")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>