drivers: serial: stm32 uart driver avoid LOG_WRN when going to sleep

When the LOG_WRN is used on stm32 uart driver it could block
execution : when pin state for sleep mode is not defined by the DTS
even if no error is raised, LOG_ msg is crashing when entering sleep mode.

Signed-off-by: Francois Ramu <francois.ramu@st.com>