xen/pci: Use PCI_MSIX_TABLE_BIR, not PCI_MSIX_FLAGS_BIRMASK

PCI_MSIX_FLAGS_BIRMASK is mis-named because the BIR mask is in the
Table Offset register, not the flags ("Message Control" per spec)
register.
Acked-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/pci/xen.c

@@ -299,7 +299,7 @@ static int xen_initdom_setup_msi_irqs(struct pci_dev *dev, int nvec, int type)
 
 			pci_read_config_dword(dev, pos + PCI_MSIX_TABLE,
 					      &table_offset);
-			bir = (u8)(table_offset & PCI_MSIX_FLAGS_BIRMASK);
+			bir = (u8)(table_offset & PCI_MSIX_TABLE_BIR);
 
 			map_irq.table_base = pci_resource_start(dev, bir);
 			map_irq.entry_nr = msidesc->msi_attrib.entry_nr;