PCI: endpoint: Improve pci_epf_type_add_cfs()

PCI: endpoint: Improve pci_epf_type_add_cfs()

pci_epf_type_add_cfs() should not be called with an unbound EPF device,
that is, an epf device with epf->driver not set. For such case, replace the
NULL return in pci_epf_type_add_cfs() with a clear ERR_PTR(-ENODEV) pointer
error return.

Link: https://lore.kernel.org/r/20230515074348.595704-2-dlemoal@kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivami <manivannan.sadhasivam@linaro.org>