Simplify the PCI bus dma tag code a bit. First, don't create a tag at

Simplify the PCI bus dma tag code a bit.  First, don't create a tag at
all for platforms that only have 32-bit bus addresses.  Second, remove
the 'tag_valid' flag from the softc.  Instead, if we don't create a
tag in pci_attach_common(), just cache the value of our parent's tag
so that we always have a valid tag to return.