PCI: Do not disregard parent resources starting at 0x0

Commit f44116ae ("PCI: Remove pci_find_parent_resource() use for
allocation") updated the logic that iterates over all bus resources and
compares them to a given resource, in order to decide whether one is the
parent of the latter.

This change inadvertently causes pci_find_parent_resource() to disregard
resources starting at address 0x0, resulting in an error such as the one
below on ARM systems whose I/O window starts at 0x0.

  pci_bus 0000:00: root bus resource [mem 0x10000000-0x3efeffff window]
  pci_bus 0000:00: root bus resource [io  0x0000-0xffff window]
  pci_bus 0000:00: root bus resource [mem 0x8000000000-0xffffffffff window]
  pci_bus 0000:00: root bus resource [bus 00-0f]
  pci 0000:00:01.0: PCI bridge to [bus 01]
  pci 0000:00:02.0: PCI bridge to [bus 02]
  pci 0000:00:03.0: PCI bridge to [bus 03]
  pci 0000:00:03.0: can't claim BAR 13 [io  0x0000-0x0fff]: no compatible bridge window
  pci 0000:03:01.0: can't claim BAR 0 [io  0x0000-0x001f]: no compatible bridge window

While this never happens on x86, it is perfectly legal in general for a PCI
MMIO or IO window to start at address 0x0, and it was supported in the code
before commit f44116ae.

Drop the test for res->start != 0; resource_contains() already checks
whether [start, end) completely covers the resource, and so it should be
redundant.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>