PCI: xgene: Add register offset to config space base address

In xgene_pcie_map_bus(), we neglected to add in the register offset when
calculating the config space address.  This means all config accesses
operated on the first four bytes of config space.

Add the register offset to the config space base address.

Also correct the xgene_pcie_map_bus() prototype to fix a compiler warning.

[bhelgaas: changelog]
Fixes: 350f8be5

 ("PCI: xgene: Convert to use generic config accessors")
Posting: http://lkml.kernel.org/r/1424214840-26498-1-git-send-email-fkan@apm.com
Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Tanmay Inamdar <tinamdar@apm.com>
Acked-by: Rob Herring <robh@kernel.org>