i2c: stm32: get rid of stm32f7_i2c_release_bus return value

Function stm32f7_i2c_release_bus is always returning 0, hence it
should be a void function.  Update the function and remove the
return value error checking code in caller functions.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>