PCI: dwc: Simplify config resource lookup

If platform_get_resource_byname("config") fails, return error immediately
and unindent the normal path.  No functional change intended.

Link: https://lore.kernel.org/r/20250117235119.712043-1-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>