Minor change to adapt Seeeduino latest config files

This version fixes a problem when compiling with old GCC as may be the case if using SeeedStudio suggested JSON configuration file for Seeeduino. This problem is not an issue in recent versions of GCC compiler.
Anyway, the updated code works with any version of GCC compiler but uses int16_t type - which may not be as pretty as "short int" from Arduino Wire perspective, but will implement the exact same GCC code.