fix LED switching on/off

In the SIMtrace and QMOD schematics the LEDs are connected to +3.3V.
Thus to switch the LED on we need to set the pin low.

Change-Id: Id8cc27e6f0b6556ba5e7ea4d254dd0fe59042a0c

firmware/libboard/common/source/led.c

@@ -16,9 +16,9 @@ static void led_set(enum led led, int on)
 	ASSERT(led < PIO_LISTSIZE(pinsLeds));
 
 	if (on)
-        	PIO_Set(&pinsLeds[led]);
-	else
         	PIO_Clear(&pinsLeds[led]);
+	else
+        	PIO_Set(&pinsLeds[led]);
 }
 
 /* LED blinking code */