x86: acpi: Refactor XSDT handling in acpi_add_table()

There is no need to have an assignment to NULL for XSDT pointer.
Therefore, no need to assign it when rsdt_address is not set.
Because of above changes we may decrease indentation level as well.

While here, drop unnecessary parentheses.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>