Change to CAD after going to STANDBY

Contrary to the data sheet, it's not possible to change to CAD mode
from e.g. RXSINGLE. Going to STANDBY first makes it happen.

Author: szotsaki

src/LoRa.cpp

@@ -306,6 +306,7 @@ void LoRaClass::sleep()
 
 void LoRaClass::cad()
 {
+    idle();
     writeRegister(REG_OP_MODE, MODE_LONG_RANGE_MODE | MODE_CAD);
 }