PCI/PM: Make pci_pme_active() ignore devices without PME support

Make pci_pme_active() ignore devices without PME support, so that
it doesn't print the "PME enabled" or "PME disabled" debug messages
for devices that don't support PME.

So that pci_pme_active() doesn't have to check pm_cap in addition
to pme_support, make pci_pm_init() clear pme_support upfront to
make sure that it will be 0 for pm_cap equal to 0.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>