serial: stm32: Use __maybe_unused instead of #if CONFIG_PM_SLEEP

Use __maybe_unused for power management related functionsinstead of

fixes: 270e5a74 ("serial: stm32: add wakeup mechanism")
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>

Link: https://lore.kernel.org/r/1560433800-12255-6-git-send-email-erwan.leray@st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>