Fixed Arduino SPI/Ethernet compile issue as described in issue #1623 (#2136)

* #1623, implementing suggested change

Splitted suggested fix issue #1623 in a header and source part. Dit not completely dive into the code.
Giving data twice as parameter feels wrong, but it compiles, and i can succesfully use the W5500 with SPI with this fix.
Doesn't compile without.

* #1623, implementing suggested change SPI.h/cpp

Splitted suggested fix issue #1623 in a header and source part. Dit not completely dive into the code.
Giving data twice as parameter feels wrong, but it compiles, and i can succesfully use the W5500 with SPI with this fix.
Doesn't compile without.

libraries/SPI/src/SPI.cpp

@@ -214,6 +214,11 @@ void SPIClass::writeBytes(uint8_t * data, uint32_t size)
     spiEndTransaction(_spi);
 }
 
+void SPIClass::transfer(uint8_t * data, uint32_t size) 
+{ 
+	transferBytes(data, data, size); 
+}
+
 /**
  * @param data void *
  * @param size uint32_t

libraries/SPI/src/SPI.h

@@ -65,10 +65,11 @@ public:
 
     void beginTransaction(SPISettings settings);
     void endTransaction(void);
-
+    void transfer(uint8_t * data, uint32_t size);
     uint8_t transfer(uint8_t data);
     uint16_t transfer16(uint16_t data);
     uint32_t transfer32(uint32_t data);
+  
     void transferBytes(uint8_t * data, uint8_t * out, uint32_t size);
     void transferBits(uint32_t data, uint32_t * out, uint8_t bits);