x86/PCI: MMCONFIG: fix region end calculation

The end of an MMCONFIG region depends on the ending bus number, not on the
number of buses the region covers.  We previously computed the wrong ending
address whenever the starting bus number was non-zero, e.g.,:

  MMCONFIG for [bus 00-1f] at [mem 0xe0000000-0xe1ffffff] (base 0xe0000000)
  MMCONFIG for [bus 20-3f] at [mem 0xe2000000-0xe1ffffff] (base 0xe0000000)

The correct regions are:

  MMCONFIG for [bus 00-1f] at [mem 0xe0000000-0xe1ffffff] (base 0xe0000000)
  MMCONFIG for [bus 20-3f] at [mem 0xe2000000-0xe3ffffff] (base 0xe0000000)

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>