Some AMD/ATI GPUs report NoSoftRst- to indicate that they perform a reset
when software transitions them from D3hot to D0, but there is no apparent
effect on the device: the monitor remains synced and the framebuffer
contents are retained.

Callers of pci_reset_function() don't necessarily have a way to validate
whether a reset was effective, so we don't want to rely on NoSoftRst if
it's known to be inaccurate.  Returning an error in such cases appears to
be the better option.  For users like vfio-pci, this allows the driver to
escalate to the bus reset interfaces.

If a device lives on the root bus, there's really no further
escalation path, so we exempt PM reset as potentially better than
nothing.
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NAlex Deucher <alexander.deucher@amd.com>