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

commit 31fc0ad47e2e0b8417616aa0f1ddcc67edf1e109 upstream.

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>
Cc: Claudius Heine <claudius.heine.ext@siemens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>