PCI PM: Return error codes from pci_pm_resume()

Currently pci_pm_resume() always returns 0, which makes the error
variable defined in there a bit pointless.  Make pci_pm_resume()
return error codes obtained from drivers' callbacks.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>