Dedupe extrakey report struct, and send functions in V-USB & LUFA (#7993)

* Dedupe extrakey report struct, and send functions in V-USB & LUFA

* Doc comment for consistency

* Wrap it in ifdef to prevent unused function error

* Do the same for ATSAM

tmk_core/common/report.h

@@ -150,6 +150,11 @@ typedef union {
 #endif
 } __attribute__((packed)) report_keyboard_t;
 
+typedef struct {
+    uint8_t  report_id;
+    uint16_t usage;
+} __attribute__((packed)) report_extra_t;
+
 typedef struct {
 #ifdef MOUSE_SHARED_EP
     uint8_t report_id;