Update Adafruit_SPIDevice.cpp

The SPI.cpp that the STM32 project uses does not have a transfer method with both a buffer and a length.  Changes to this file uses a transfer method that just needs a single 8 bit character.

Adafruit_SPIDevice.cpp

@@ -116,7 +116,8 @@ void Adafruit_SPIDevice::transfer(uint8_t *buffer, size_t len) {
 #ifdef SPARK
     _spi->transfer(buffer, buffer, len, NULL);
 #else
-    _spi->transfer(buffer, len);
+    for(i = 0; i < len; i++ )
+      _spi->transfer(buffer[i]);
 #endif
 
     return;