ACPI: PCI: always use the PCI INTx pin values, not the _PRT ones

This patch changes pci_irq.c to always use PCI INTx pin encodings
instead of a mix of PCI and _PRT encodings.

The PCI INTx pin numbers from the PCI_INTERRUPT_PIN config register
are 0=device doesn't use interrupts, 1=INTA, ..., 4=INTD.  But the
_PRT table uses 0=INTA, ..., 3=INTD.

This patch converts the _PRT encoding to the PCI encoding immediately
when we add a _PRT entry to the global list.  All the rest of the
code can then use the PCI encoding consistently.

The point of this is to make the interrupt swizzling look the same
as on other architectures, so someday we can unify them.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>