PCI: Assign bus numbers present in EA capability for bridges

The "Enhanced Allocation (EA) for Memory and I/O Resources" ECN, approved
23 October 2014, sec 6.9.1.2, specifies a second DW in the capability for
type 1 (bridge) functions to describe fixed secondary and subordinate bus
numbers.  This ECN was included in the PCIe r4.0 spec, but sec 6.9.1.2 was
omitted, presumably by mistake.

Read fixed bus numbers from the EA capability for bridges.

Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
[bhelgaas: add pci_ea_fixed_busnrs() return value]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>