serial: stm32: select pinctrl state in each suspend/resume function

Select either pinctrl sleep state in suspend function or default state in
resume function.

Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>