PCI: Catch attempts to disable already-disabled devices

Warn when disabling a device that has already been disabled.

[bhelgaas: message wording]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>