PCI/DPC: Skip DPC event if device is not present

The DPC interupt may be executed on a device that is being removed.  Skip
queuing event handling if the status is all 1's, which should be seen only
if the device is not present.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>