[sam] Fix SAMx7x ADC readChannel() if previous data has not been read

Clear data ready flag and do not return previous result if it has not
been read from the LCDR register. When using conversion sequences data
is normally read from the CDR register which does not clear the data
ready flag.

src/modm/platform/adc/samg/adc.hpp.in

@@ -251,6 +251,8 @@ public:
 	{
 		if (!setChannel(channel)) return 0;
 
+		// clear data ready by reading last value register
+		getValue();
 		startConversion();
 		while (!isConversionFinished()) {}