PCI: PCIe portdrv: Add kerneldoc comments to some core functions

Add kerneldoc comments to some functions in
drivers/pci/pcie/portdrv_core.c, since the code in there is not
easy to follow without any additional description.

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