I2C transfer functions return number of successful operations (on success).

Do not return the received positive return code but instead return 0 on
success. The users of write_reg function already use this logic.
Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>