i2c: stm32: fix comment and remove unused AUTOEND bit

Comment within stm32_i2c_message_start is misleading, indicating
that AUTOEND bit is setted while it is actually cleared.
Moreover, the bit is actually never setted so there is no need
to clear it hence get rid of this bit clear and the bit macro
as well.

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>