PCI: use config space encoding in pci_get_interrupt_pin()

This patch makes pci_get_interrupt_pin() return values encoded
the same way as the "Interrupt Pin" value in PCI config space,
i.e., 1=INTA, ..., 4=INTD.

pirq_bios_set() is the only in-tree caller of pci_get_interrupt_pin()
and pci_get_interrupt_pin() is not exported.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: hpa@zytor.com
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>