[HID] added 'const' qualifier in HID initialization

See arduino/Arduino#3840 (comment)

Author: cmaglie

Diff for: arduino/avr/libraries/HID/HID.h

@@ -47,7 +47,7 @@
 class HIDDescriptorListNode {
 public:
   HIDDescriptorListNode *next = NULL;
-  HIDDescriptorListNode(const void *d, uint16_t l) : data(d), length(l) { }
+  HIDDescriptorListNode(const void *d, const uint16_t l) : data(d), length(l) { }
 
   const void* data;
   uint16_t length;