PCI: Do not allocate more buses than available in parent

One can ask more buses to be reserved for hotplug bridges by passing
pci=hpbussize=N in the kernel command line.  If the parent bus does not
have enough bus space available we incorrectly create child bus with the
requested number of subordinate buses.

In the example below hpbussize is set to one more than we have available
buses in the root port:

  pci 0000:07:00.0: [8086:1578] type 01 class 0x060400
  pci 0000:07:00.0: scanning [bus 00-00] behind bridge, pass 0
  pci 0000:07:00.0: bridge configuration invalid ([bus 00-00]), reconfiguring
  pci 0000:07:00.0: scanning [bus 00-00] behind bridge, pass 1
  pci_bus 0000:08: busn_res: can not insert [bus 08-ff] under [bus 07-3f] (conflicts with (null) [bus 07-3f])
  pci_bus 0000:08: scanning bus
  ...
  pci_bus 0000:0a: bus scan returning with max=40
  pci_bus 0000:0a: busn_res: [bus 0a-ff] end is updated to 40
  pci_bus 0000:0a: [bus 0a-40] partially hidden behind bridge 0000:07 [bus 07-3f]
  pci_bus 0000:08: bus scan returning with max=40
  pci_bus 0000:08: busn_res: [bus 08-ff] end is updated to 40

Instead of allowing this, limit the subordinate number to be less than or
equal the maximum subordinate number allocated for the parent bus (if it
has any).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
[bhelgaas: remove irrelevant dmesg messages]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>