Fixes the following W=1 kernel build warning(s):

 drivers/i2c/busses/i2c-stm32f4.c:321: warning: expecting prototype for stm32f4_i2c_write_ byte()(). Prototype was for stm32f4_i2c_write_byte() instead
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>