GSM: add end() implementation

Author: pennam

libraries/GSM/src/GSM.cpp

@@ -110,7 +110,7 @@ bool arduino::GSMClass::isCmuxEnable() {
 }
 
 void arduino::GSMClass::end() {
-
+  _device->shutdown();
 }
 
 int arduino::GSMClass::disconnect() {

libraries/GSM/src/GSM.h

@@ -95,6 +95,9 @@ class GSMClass : public MbedSocketClass {
      */
   int disconnect(void);
 
+  /*
+   * Reset internal state machine in order to be ready to reconnect again.
+   */
   void end(void);
 
   unsigned long getTime();