ACPI / PM: Remove redundant power manageable check from acpi_bus_set_power()

Now that acpi_device_set_power() checks whether or not the given
device is power manageable, it is not necessary to do this check in
acpi_bus_set_power() any more, so remove it.

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