PCI: xilinx-nwl: Avoid crashing if configuring when the link is down

PCI: xilinx-nwl: Avoid crashing if configuring when the link is down

The ECAM will return a slave error if we access non-root devices while
the link is down. Add a check for this like Linux does so we don't
crash.

Fixes: 2f5ad77cfea ("PCI: zynqmp: Add ZynqMP NWL PCIe root port driver")
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed-by: Stefan Roese <stefan.roese@mailbox.org>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20260106220853.731358-1-sean.anderson@linux.dev