ACPI / x86: boot: Use INVALID_ACPI_IRQ instead of 0 for acpi_sci_override_gsi

0 is valid hardware interrupt which might be in some cases overridden.
Due to this, switch to INVALID_ACPI_IRQ to mark SCI override not set.

While here, change the type of the variable from int to u32 to match
the GSI type used in the rest of the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>