ACPI / x86: boot: Propagate error code in acpi_gsi_to_irq()

acpi_get_override_irq() followed by acpi_register_gsi() returns negative
error code on failure.

Propagate it from acpi_gsi_to_irq() to callers.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[ rjw : Subject/changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>