[taiko] improve reading adc result

from: arduino/ArduinoCore-avr#345

7.3.0-atmel3.6.1-arduino7 gcc fails to optimize separate reading from
ADCL and ADCH. It produces additionally three eor commands or in some
cases two mov commands in the assembly code. These commands swap
register contents before store them to data area. So they are
completely unnecessary. Reading ADC result with ADC macro fixes it and
gcc generates the right code..

Author: unrevre

nonblock_read.c

@@ -37,29 +37,21 @@ void analogPrepareRead(uint8_t pin)
 #endif
 #endif
 
-#if defined(ADCSRA) && defined(ADCL)
+#if defined(ADCSRA) && defined(ADC)
 	// start the conversion
 	sbi(ADCSRA, ADSC);
 #endif
 }
 
 int analogReadData(void)
 {
-#if defined(ADCSRA) && defined(ADCL)
+#if defined(ADCSRA) && defined(ADC)
 	// ADSC is cleared when the conversion finishes
 	while (bit_is_set(ADCSRA, ADSC));
 
-	// we have to read ADCL first; doing so locks both ADCL
-	// and ADCH until ADCH is read.  reading ADCL second would
-	// cause the results of each conversion to be discarded,
-	// as ADCL and ADCH would be locked when it completed.
-	uint8_t low, high;
-
-	low  = ADCL;
-	high = ADCH;
-
-	// combine the two bytes
-	return (high << 8) | low;
+	// ADC macro takes care of reading ADC register.
+	// avr-gcc implements the proper reading order: ADCL is read first.
+	return ADC;
 #else
 	// we dont have an ADC, return 0
 	return 0;