SAM Wire: fixed NACK/timeout behavior of requestFrom() and available()

"readed" is no longer incremented in requestFrom() if
TWI_WaitByteReceived() gets a NACK or times out. This corrects the
behavior (return values) of requestFrom() and available() to match the
Arduino reference. Fixes arduino/Arduino#1311
Merge pull request #4 from kevin-pololu/due-wire-available

SAM Wire: fixed NACK/timeout behavior of requestFrom() and available()