ACPI / x86: boot: Swap variables in condition in acpi_register_gsi_ioapic()

For better readability compare input to something considered settled
down. Additionally move it to one line (while it's slightly longer
80 characters it makes readability better).

No functional change intended.

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