write method parameter const-ness

fix adafruit/Adafruit_FRAM_SPI#25

Author: yilungao

Adafruit_SPIDevice.cpp

@@ -295,8 +295,8 @@ void Adafruit_SPIDevice::endTransaction(void) {
  *    @return Always returns true because there's no way to test success of SPI
  * writes
  */
-bool Adafruit_SPIDevice::write(uint8_t *buffer, size_t len,
-                               uint8_t *prefix_buffer, size_t prefix_len) {
+bool Adafruit_SPIDevice::write(const uint8_t *buffer, size_t len,
+                               const uint8_t *prefix_buffer, size_t prefix_len) {
   if (_spi) {
     _spi->beginTransaction(*_spiSetting);
   }
@@ -402,7 +402,7 @@ bool Adafruit_SPIDevice::read(uint8_t *buffer, size_t len, uint8_t sendvalue) {
  *    @return Always returns true because there's no way to test success of SPI
  * writes
  */
-bool Adafruit_SPIDevice::write_then_read(uint8_t *write_buffer,
+bool Adafruit_SPIDevice::write_then_read(const uint8_t *write_buffer,
                                          size_t write_len, uint8_t *read_buffer,
                                          size_t read_len, uint8_t sendvalue) {
   if (_spi) {

Adafruit_SPIDevice.h

@@ -77,9 +77,9 @@ class Adafruit_SPIDevice {
 
   bool begin(void);
   bool read(uint8_t *buffer, size_t len, uint8_t sendvalue = 0xFF);
-  bool write(uint8_t *buffer, size_t len, uint8_t *prefix_buffer = NULL,
+  bool write(const uint8_t *buffer, size_t len, const uint8_t *prefix_buffer = NULL,
              size_t prefix_len = 0);
-  bool write_then_read(uint8_t *write_buffer, size_t write_len,
+  bool write_then_read(const uint8_t *write_buffer, size_t write_len,
                        uint8_t *read_buffer, size_t read_len,
                        uint8_t sendvalue = 0xFF);
   bool write_and_read(uint8_t *buffer, size_t len);