mailbox: stm32-ipcc: remove useless device_init_wakeup call

If the "wakeup-source" property does not exist there is no need to call
device_init_wakeup("false") at probe.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>