ACPI / PM: Handle missing _PSC in acpi_bus_update_power()

If _PS0 is defined for an ACPI device node, but _PSC isn't and
the device node doesn't use power resources for power management,
acpi_bus_update_power() will fail to update the power state of it,
because acpi_device_get_power() returns ACPI_STATE_UNKNOWN in that
case.

To handle that situation make acpi_bus_update_power() follow
acpi_bus_init_power() and try to force the given device node into
power state D0.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>