Revert "PCI: Distribute available resources for root buses, too"

This reverts commit e96e27fc.

Jonathan reported that this commit broke this topology, where all the space
available on bus 02 was assigned to the 02:00.0 bridge window, leaving none
for the e1000 device at 02:00.1:

  pci 0000:00:04.0: bridge window [mem 0x10200000-0x103fffff] to [bus 02-04]
  pci 0000:02:00.0: bridge window [mem 0x10200000-0x103fffff] to [bus 03-04]
  pci 0000:02:00.1: BAR 0: failed to assign [mem size 0x00020000]
  e1000 0000:02:00.1: can't ioremap BAR 0: [??? 0x00000000 flags 0x0]

Link: https://lore.kernel.org/r/20221014124553.0000696f@huawei.com


Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>