PCI: Clean up MSI/MSI-X capability #defines

This doesn't change any existing symbols, but it puts them in logical
order and uses explicit masks instead of shifts, like the rest of the
file.

It also adds new symbols for PCI_MSIX_TABLE_BIR,
PCI_MSIX_TABLE_OFFSET, PCI_MSIX_PBA_BIR, and PCI_MSIX_PBA_OFFSET to
replace the mis-named PCI_MSIX_FLAGS_BIRMASK (the BAR index fields
are part of the Table and PBA registers, not the flags register).

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