PCI: designware: Simplify pcie_host_ops.readl_rc() and .writel_rc() interfaces

The struct pcie_host_ops.readl_rc() and .writel_rc() function pointers
allow a driver to override the default DesignWare register accessors.

Make the signature of the override functions the same as the default
accessors.  This makes the default dw_pcie_readl_rc() and the corresponding
override more structurally similar: both will compute the final register
address with "pp->dbi_base + reg".  Previously dw_pcie_readl_rc() computed
the address and passed it to the override.

No functional change intended.

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