Ages ago, drivers could return values greater than zero from their probe
function and this would be regarded as success.

But after f3ec4f87 ("PCI: change device runtime PM settings for probe
and remove") and 967577b0 ("PCI/PM: Keep runtime PM enabled for unbound
PCI devices"), we set dev->driver to NULL if the driver's probe function
returns a value greater than zero.

__pci_device_probe() treats this as success, and drivers can still mostly
work even with dev->driver == NULL, but PCI power management doesn't work,
and we don't call the driver's remove function on rmmod.

To help catch these driver problems, issue a warning in this case.

[bhelgaas: changelog]
Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>