[POWERPC] Do not write virq back to PCI config space

- Drivers will not rely on the PCI config space value, as they've
  already been conditioned to rely on the irq field in "struct pci_dev".

- The virq value may not be < 256 as it has been remapped.

- The PCI config space should reflect the hardware configuration, which
  is not being changed.  We are only creating a virtual irq mapping that
  exists in the kernel only. One would never expect the PCI hardware to
  generate the "virq" interrupt.

Signed-off-by: Michal Ostrowski <mostrows@watson.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>