serial: stm32: fix wakeup source initialization

Fixes dedicated_irq_wakeup issue and deactivated uart as wakeup source by
default.

Fixes: 270e5a74 ("serial: stm32: add wakeup mechanism")
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>