PCI/PM: Expand description of pci_set_power_state()

Add two reasons for returning 0 value to the description of
pci_set_power_state() to include the cases when:

  - the transition is to D1 or D2 but D1 and D2 are not supported
  - the transition is to D3 but D3 is not supported

Signed-off-by: Piotr Gregor <piotrgregor@rsyncme.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>