serial: stm32: clock disabling management

Keep the clock enabled at the end of stm32_init_port
but disable it in stm32_serial_remove.
Note that stm32_pm function is there to manage the
clock at runtime.

Signed-off-by: Gerald Baeza <gerald.baeza@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>