[ACPI] don't complain about PCI root bridges without _SEG

There are lots of single-PCI-segment machines that don't
supply _SEG for the root bridges.  The PCI root bridge driver
silently assumes the segment to be zero in this case,
so glue.c shouldn't complain either.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>