PCI: exynos: Pass device-specific struct to internal functions

Only interfaces used from outside the driver, e.g., those called by the
DesignWare core, need to accept pointers to the generic struct pcie_port.
Internal interfaces can accept pointers to the device-specific struct,
which makes them more straightforward.  No functional change intended.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>