PCI: Use cached MSI cap while enabling MSI interrupts

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

[bhelgaas: removed unrelated msi_control_reg() changes]
Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>