PCI: portdrv: Deduplicate PM callback iterator

Replace suspend_iter() and resume_iter() with a single function pm_iter()
to allow addition of port service callbacks for further power management
phases without having to add another iterator each time.

No functional change intended.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>