PCI: altera: Rework config accessors for use without a struct pci_bus

Rework configs accessors so a future patch can use them in _probe() with
struct altera_pcie instead of struct pci_bus.

Signed-off-by: Ley Foon Tan <lftan@altera.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>