PCI: Use cached MSI-X cap while enabling MSI-X

The patch uses the cached MSI-X capability offset in
pci_dev instead of reading it from config space when enabling
MSI-X interrupts.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>