[PUSB] callbacks are now pure virtual methods

This change allows the compiler to handle callbacks resolution.

Callbacks now must be implemented on the class that extends
PUSBListNode and this is forced by compiler by means of pure
virtual methods.

Also the calls to HID.interface() and HID.endpoint() can now
be simplified to interface() and endpoint() respectively since
the methods are no more static.

Author: cmaglie

hardware/arduino/avr/cores/arduino/PluggableUSB.h

@@ -28,9 +28,6 @@
 class PUSBListNode {
 public:
   PUSBListNode() { }
-  bool (*setup)(USBSetup& setup, uint8_t i);
-  int (*getInterface)(uint8_t* interfaceNum);
-  int (*getDescriptor)(int8_t t);
   int8_t numEndpoints;
   int8_t numInterfaces;
   uint8_t *endpointType;
@@ -39,6 +36,10 @@ class PUSBListNode {
   inline int8_t endpoint()   const { return pluggedEndpoint; }
 
 protected:
+  virtual bool setup(USBSetup& setup, uint8_t i) = 0;
+  virtual int getInterface(uint8_t* interfaceNum) = 0;
+  virtual int getDescriptor(int8_t t) = 0;
+
   uint8_t pluggedInterface;
   int8_t pluggedEndpoint;