PCI: designware: Make "num-lanes" an optional DT property

Currently "num-lanes" is read in dw_pcie_host_init(), but it is only used
if we call dw_pcie_setup_rc() while bringing up the link.  If the link has
already been brought up by firmware, we need not call dw_pcie_setup_rc(),
and "num-lanes" is unnecessary.

Only complain about "num-lanes" if we actually need it and we didn't find a
valid value.

[bhelgaas: changelog]
Signed-off-by: Gabriele Paoloni <gabriele.paoloni@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>