The patch caches the MSI and MSI-X capability offset in PCI device
(struct pci_dev) so that we needn't read it from the config space
upon enabling or disabling MSI or MSI-X interrupts.

[bhelgaas: moved pm_cap size change to separate patch]
Signed-off-by: NGavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>