x86: acpi: Remove header length check when writing tables

Before moving 'current' pointer during ACPI table writing, we always
check the table length to see if it is larger than the table header.
Since our purpose is to generate valid tables, the check logic is
always true, which can be avoided.

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