Wire: write buffer API should have const buffer arg

[sandeepmistry]

Comparing the Wire library with SAMD:

https://github.com/arduino/ArduinoCore-nRF528x-mbedos/blob/master/libraries/Wire/Wire.h#L48

virtual size_t write(uint8_t* data, int len);

to

size_t write(const uint8_t * data, size_t quantity);

https://github.com/arduino/ArduinoCore-samd/blob/master/libraries/Wire/Wire.h