Fixes for class C activation and deactivation (#1325)

* Deactivate radio on a switch from class C to A

When the MAC switches from class C to class A, we must update one of the
internal state variables to make sure that the radio is not put into the
continuous RX mode again in LoRaMacProcess.

* Reactivate radio for continuous RX when starting in class C

* Put radio to sleep in LoRaMacStop if in class C

If the MAC is operating in class C and the application invokes
LoRaMacStop, make sure the radio will not remain receiving continuously,
draining batteries. LoRaMacStart will switch the radio into a continuous
RX mode again.

Author: janakj

src/mac/LoRaMac.c

@@ -2018,6 +2018,9 @@ static LoRaMacStatus_t SwitchClass( DeviceClass_t deviceClass )
         {
             if( deviceClass == CLASS_A )
             {
+                // Reset RxSlot to NONE
+                MacCtx.RxSlot = RX_SLOT_NONE;
+
                 Nvm.MacGroup2.DeviceClass = deviceClass;
 
                 // Set the radio into sleep to setup a defined state
@@ -3827,13 +3830,18 @@ LoRaMacStatus_t LoRaMacInitialization( LoRaMacPrimitives_t* primitives, LoRaMacC
 LoRaMacStatus_t LoRaMacStart( void )
 {
     MacCtx.MacState = LORAMAC_IDLE;
+    UpdateRxSlotIdleState();
     return LORAMAC_STATUS_OK;
 }
 
 LoRaMacStatus_t LoRaMacStop( void )
 {
     if( LoRaMacIsBusy( ) == false )
     {
+        if( Nvm.MacGroup2.DeviceClass == CLASS_C )
+        {
+            Radio.Sleep( );
+        }
         MacCtx.MacState = LORAMAC_STOPPED;
         return LORAMAC_STATUS_OK;
     }