PCI: designware: Simplify dw_pcie_readl_unroll(), dw_pcie_writel_unroll()

dw_pcie_readl_unroll() and dw_pcie_writel_unroll() duplicate what
dw_pcie_readl_rc() and dw_pcie_writel_rc() already do, so call them
directly.

[bhelgaas: reworked into patch series]
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>