PCI: Rationalize pci_ats_queue_depth() error checking

We previously returned -ENODEV for devices that don't support ATS (except
that we always returned 0 for VFs, whether or not they support ATS).

For consistency, always return -EINVAL (not -ENODEV) if the device doesn't
support ATS.  Return zero for VFs that support ATS.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Joerg Roedel <jroedel@suse.de>