acpi_unregister_gsi() should "undo" what acpi_register_gsi() does.

On systems that have legacy interrupts, acpi_unregister_gsi erroneously calls
iosapci_unregister_intr() which is wrong to do and causes a loud warning.

acpi_unregister_gsi() should just return in these cases.
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>