x86/pci/intel_mid_pci: Use proper constants for irq polarity

polarity = 0 means active high. Not really intuitive, so people add
comments to it instead of just using a self explaining constant.

Use the IOAPIC_POL_ constants and get rid of those horrible to read
tail comments.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>