PCI: endpoint: Fix NULL pointer dereference error when CONFIGFS is disabled

commit ef1433f7 ("PCI: endpoint: Create configfs entry for each
pci_epf_device_id table entry") while adding configfs entry for each
pci_epf_device_id table entry introduced a NULL pointer dereference error
when CONFIG_PCI_ENDPOINT_CONFIGFS is not enabled.

Fix it here.

Fixes: ef1433f7

 ("PCI: endpoint: Create configfs entry for each
pci_epf_device_id table entry")
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
[lorenzo.pieralisi: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>