Configure RS485 mode during port initialization.

Fixes: 1bcda09d ("serial: stm32: add support for RS485 hardware control mode")
Signed-off-by: NBich Hemon <bich.hemon@st.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>