PCI: Drop msi_control_reg() macro and use PCI_MSI_FLAGS directly

Note the error in pci_msix_table_size() -- we used PCI_MSI_FLAGS to
locate the PCI_MSIX_FLAGS word.  No actual breakage because PCI_MSI_FLAGS
and PCI_MSIX_FLAGS happen to be the same.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>