scd4x: implement startLowPower, remove unsupported stopLowPower

include/driver/scd4x.h

@@ -24,7 +24,6 @@ class SCD4x {
 		void stop();
 
 		void startLowPower();
-		void stopLowPower();
 
 		void read();
 };

src/driver/scd4x.cc

@@ -25,6 +25,13 @@ void SCD4x::stop()
 	i2c.xmit(address, 2, txbuf, 0, rxbuf);
 }
 
+void SCD4x::startLowPower()
+{
+	txbuf[0] = 0x21;
+	txbuf[1] = 0xac;
+	i2c.xmit(address, 2, txbuf, 0, rxbuf);
+}
+
 void SCD4x::read()
 {
 	txbuf[0] = 0xec;