PCI: Make pcie_downstream_port() available outside of access.c

PCI: Make pcie_downstream_port() available outside of access.c

pcie_downstream_port() is useful in other places where code needs to
determine whether the PCIe port is downstream so make it available outside
of access.c.

Link: https://lore.kernel.org/r/20190822085553.62697-1-mika.westerberg@linux.intel.com
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>