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: f55fee56 ("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>