pci: fsl: Do not access PCI BAR0 register of PCIe Root Port

Freescale PCIe Root Port has PEXCSRBAR register at position of PCI BAR0.
PCIe Root Port does not have any PCIe memory, so returns zero when trying
to read from PCIe Root Port BAR0 and ignore any writes.

Signed-off-by: Pali Rohár <pali@kernel.org>