i2c: stm32: remove unused stop parameter in start & reload handling

Functions stm32_i2c_message_start and stm32_i2c_handle_reload
both get a stop boolean indicating if the transfer should end with
a STOP or not.  However no specific handling is needed in those
functions hence remove the parameter.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>