Update SPI.h - Add transfer_out

An alternative "output only" transfer that doesn't destroy the source buffer with the bytes returned from the slave device.
Some code simplifications and const enforcement of input variables
Tested in ATMEGA328p (UNO)
* no speed changes noticed (comparing the the original function)
* writing/reading to an SPI RAM returns the correct values

Author: pamribeirox

hardware/arduino/avr/libraries/SPI/SPI.h

@@ -255,6 +255,21 @@ class SPIClass {
     while (!(SPSR & _BV(SPIF))) ;
     *p = SPDR;
   }
+  /*
+   * An alternative "output only" transfer that doesn't destroy the
+   * contents of the source buffer with the bytes returned from the
+   * slave device. 
+   * Some code simplifications and const enforcement
+   * by Pedro Ribeiro pamribeirox@gmail.com
+   */
+  inline static void transfer_out(const void *buf, size_t count) {
+    if (count == 0) return;
+    uint8_t *p = (uint8_t *)buf;
+    while (count-- > 0) {
+      SPDR = *p++;
+      while (!(SPSR & _BV(SPIF))) ;
+    }
+  }
   // After performing a group of transfers and releasing the chip select
   // signal, this function allows others to access the SPI bus
   inline static void endTransaction(void) {