PCI: Enumerate subordinate buses, not devices, in pci_bus_get_depth()

Normally, on one PCI bus there would be more devices than bridges.  When
calculating the depth of a PCI bus, it would be more time efficient to
enumerating through the child buses instead of the child devices.

Also by doing so, the code seems more self explaining.  Previously, it went
through the devices and checked whether a bridge introduced a child bus or
not, which needs more background knowledge to understand it.

This patch calculates the depth by enumerating the bus hierarchy.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>