serial: stm32: fix multi-ports management

Correct management of multi-ports. Each port has
its own last residue value and its own alias.

Signed-off-by: Gerald Baeza <gerald.baeza@st.com>
Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>