PCI: dwc: Don't assume the ops in dw_pcie always exist

Some dwc-based device drivers, especially host-only drivers, may work well
with the default read_dbi/write_dbi/link_up implementations in
pcie-designware.c, so remove the assumption that every driver implements
them to simplify those drivers.

Link: https://lore.kernel.org/r/20210128144258.10329aa4@xhacker.debian


Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>