Wire: Let requestFrom return the number of available bytes

[matthijskooijman]

Previously, it would return the number of requested bytes, even if the
slave returned a NAK on the address byte, preventing libraries to detect
such a NAK. For example, the SparkFun HTU21D library would fail to read
humidity, because the "I'm not ready measuring"-NAK would look like a
successful read, but with dummy data.

This matches the return value with that of the official Arduino versions
and the documentation at
https://www.arduino.cc/en/Reference/WireRequestFrom

[GrumpyOldPizza]

ACK. I recall having coded that specifically as to one of the cores (must have been ArduinoCore-samd at the time) had exactly those semantics.

[matthijskooijman]

Yup, seems they fixed it for SAMD here-ish (https://github.com/adafruit/ArduinoCore-samd/blob/master/libraries/Wire/Wire.cpp#diff-df1cdf716a3ab485d49151a55cbbd816f3689b57dc5999174d66f9d8f0241bac) and it now looks like this: https://github.com/adafruit/ArduinoCore-samd/blob/0b160161de695ba9e5b0ce223f9e1181224800d9/libraries/Wire/Wire.cpp#L65-L98