Set proper default pins for SPI

Some boards have different default pins for SPI

Thanks @ladyada

Author: me-no-dev

Diff for: libraries/SPI/src/SPI.h

@@ -51,7 +51,7 @@ class SPIClass
 
 public:
     SPIClass(uint8_t spi_bus=HSPI);
-    void begin(int8_t sck=-1, int8_t miso=-1, int8_t mosi=-1, int8_t ss=-1);
+    void begin(int8_t sck=SCK, int8_t miso=MISO, int8_t mosi=MOSI, int8_t ss=-1);
     void end();
 
     void setHwCs(bool use);