Optimized buffer transfer to not extend public API

SPI.h

@@ -1264,12 +1264,8 @@ public:
     }
 
 	void inline transfer(void *buf, size_t count) {transfer(buf, buf, count);}
-    void inline transfer16(void *buf, size_t count) {transfer16(buf, buf, count);}
-    void inline transfer32(void *buf, size_t count) {transfer32(buf, buf, count);}
 	void setTransferWriteFill(uint8_t ch ) {_transferWriteFill = ch;}
 	void transfer(const void * buf, void * retbuf, size_t count);
-    void transfer16(const void * buf, void * retbuf, size_t count);
-    void transfer32(const void * buf, void * retbuf, size_t count);
 
 	// Asynch support (DMA )
 #ifdef SPI_HAS_TRANSFER_ASYNC
@@ -1392,6 +1388,12 @@ private:
 	DMAChannel    *_dmaRX = nullptr;
 	EventResponder *_dma_event_responder = nullptr;
 #endif
+
+	// Optimized buffer transfer
+	void inline transfer16(void *buf, size_t count) {transfer16(buf, buf, count);}
+	void inline transfer32(void *buf, size_t count) {transfer32(buf, buf, count);}
+	void transfer16(const void * buf, void * retbuf, size_t count);
+	void transfer32(const void * buf, void * retbuf, size_t count);
 };