PCI: PM: Make pci_choose_state() call pci_target_state()

The pci_choose_state() and pci_target_state() implementations are
somewhat divergent without a good reason, because they are used
for similar purposes.

Change the pci_choose_state() implementation to use pci_target_state()
internally except for transitions to the working state of the system
in which case it is expected to return D0.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Ferry Toth <fntoth@gmail.com>