device_pm: correct nop documented behavior

The device_pm_control_nop function is documented to always return zero
regardless of operation.  However, when device_get_power_state() is
invoked with this control function it returns success leaving the
output parameter state unmodified, which may not be a valid device
state.

Document and implement that the nop control function returns -ENOTSUP
always.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>