x86: ACPI: Make mp_config_acpi_gsi() a void function

Because the return value of mp_config_acpi_gsi() is not use, change it
into a void function.

Signed-off-by: Li kunyu <kunyu@nfschina.com>
[ rjw: Subject and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>