PCI: designware: Swap order of dw_pcie_writel_unroll() reg/val arguments

Swap order of dw_pcie_readl_unroll() arguments to match the "dev, pos, val"
order used by pci_write_config_word() and other drivers.  No functional
change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>