ACPI / PCI: Simplify acpi_penalize_isa_irq()

acpi_penalize_isa_irq() can be written in fewer lines of code,
so do that.  No functional change.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Works-for: Andy Shevchenko <andy.shevchenko@gmail.com>