PCI: Remove unnecessary list_empty(&pci_pme_list) check

list_for_each_entry() handles empty lists just fine, so there's no need to
check whether the list is empty first.

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>