PCI: qcom-ep: Check platform_get_resource_byname() return value

If platform_get_resource_byname() fails, 'mmio_res' will be set to NULL
pointer, which causes a NULL pointer dereference when it is used in
qcom_pcie_perst_deassert().

Check the return value to prevent it.

Link: https://lore.kernel.org/r/20220429080740.1294797-1-yangyingliang@huawei.com
Fixes: f55fee5 ("PCI: qcom-ep: Add Qualcomm PCIe Endpoint controller driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>

drivers/pci/controller/dwc/pcie-qcom-ep.c

@@ -474,6 +474,11 @@ static int qcom_pcie_ep_get_io_resources(struct platform_device *pdev,
 
 	pcie_ep->mmio_res = platform_get_resource_byname(pdev, IORESOURCE_MEM,
 							 "mmio");
+	if (!pcie_ep->mmio_res) {
+		dev_err(dev, "Failed to get mmio resource\n");
+		return -EINVAL;
+	}
+
 	pcie_ep->mmio = devm_pci_remap_cfg_resource(dev, pcie_ep->mmio_res);
 	if (IS_ERR(pcie_ep->mmio))
 		return PTR_ERR(pcie_ep->mmio);