HID: Renamed fields in HIDDescriptorListNode and HID_Descriptor

In particular HIDDescriptorListNode.cb has been renamed to
HIDDescriptorListNode.descriptor because it contains decriptor data
and not callbacks.

Moreover the HID_Descriptor.descriptor field has been renamed
to HID_Descriptor.data so the structure has now two fields length
and data.

   typedef struct __attribute__((packed)) {
     uint16_t length;
     const void* data;
   } HID_Descriptor;

   class HIDDescriptorListNode {
   public:
     HIDDescriptorListNode *next = NULL;
     const HID_Descriptor *descriptor;
     HIDDescriptorListNode(const HID_Descriptor *d) : descriptor(d) { }
   };

This imply a change in the use of the node from:

  node->cb->lenght
  node->cd->descriptor

to

  node->descriptor->length
  node->descriptor->data

Author: cmaglie

src/Keyboard.cpp

@@ -62,11 +62,11 @@ static const uint8_t _hidReportDescriptor[] PROGMEM = {
 
 Keyboard_::Keyboard_(void) 
 {
-	static HID_Descriptor cb = {
+	static HID_Descriptor descriptor = {
 		.length = sizeof(_hidReportDescriptor),
-		.descriptor = _hidReportDescriptor,
+		.data   = _hidReportDescriptor,
 	};
-	static HIDDescriptorListNode node(&cb);
+	static HIDDescriptorListNode node(&descriptor);
 	HID.AppendDescriptor(&node);
 }