Fix spi.transfer() "is ambiguous" issue

Author: board707

STM32F1/libraries/SPI/src/SPI.h

@@ -276,9 +276,9 @@ class SPIClass {
 
     // Some libraries (like recent Adafruit graphics libraries) require
     // the write function be availabe under the name transfer, so here it is:
-    /*inline void transfer(const void * buffer, size_t length) {
+    inline void transfer(const void * buffer, uint32 length) {
      write(buffer, (uint32)length);
-    }*/
+    }
 
     /**
      * @brief Transmit a byte, then return the next unread byte.