PCI: Handle IORESOURCE_PCI_FIXED when assigning resources

The new Enhanced Allocation (EA) capability support (patches to follow)
creates resources with the IORESOURCE_PCI_FIXED set.  During resource
assignment in pci_bus_assign_resources(), IORESOURCE_PCI_FIXED resources
are not given a parent.  This, in turn, causes pci_enable_resources() to
fail with a "not claimed" error.

So, in __pci_bus_assign_resources(), for IORESOURCE_PCI_FIXED resources,
try to request the resource from a parent bus.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Sean O. Stalley <sean.stalley@intel.com>