i2c: use uint8_t when appropriate and remove char

Object APIs used signed chars for some operations which are not appropriate
when using i2c.

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
i2c_HMC5883L.c: fix type in i2c buffer

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
rgblcd.js: add i2c example similar to python

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
rgblcd.py: update example to use only writeReg()

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>