Arduino Zero & Due lack SPI.transfer16()

[PaulStoffregen]

The transfer16() function was added to the SPI library quite some time ago. It's been documented on the website.

https://www.arduino.cc/en/Reference/SPITransfer

It works on AVR boards, but Arduino Zero & Due don't have transfer16() in their SPI libraries yet.

[cmaglie]

Fixed with #4081 and arduino/ArduinoCore-samd#59