Fix auto-idle.

Powering on even with a heater set to zero doesn't work, as
temperature driven heaters are set to zero all the time. So,
the PSU was running all the time.

The bug was introduced with 569e3d504aafd1ffc8b279b5d7092970cef72287

clock.c

@@ -22,7 +22,6 @@
 	called from clock_10ms(), do not call directly
 */
 static void clock_250ms(void) {
-	#ifndef	NO_AUTO_IDLE
   if (heaters_all_zero()) {
 		if (psu_timeout > (30 * 4)) {
 			power_off();
@@ -36,7 +35,6 @@ static void clock_250ms(void) {
 			SREG = save_reg;
 		}
 	}
-	#endif
 
 	ifclock(clock_flag_1s) {
 		if (DEBUG_POSITION && (debug_flags & DEBUG_POSITION)) {

heater.c

@@ -438,9 +438,8 @@ void heater_set(heater_t index, uint8_t value) {
 			*(heaters[index].heater_port) &= ~MASK(heaters[index].heater_pin);
 	}
 
-  // Do this even when the heater is set to
-  // zero to deal with long cool down phases.
-  power_on();
+  if (value)
+    power_on();
 }
 
 /** \brief check wether all heaters are off