rtc: stm32: manage the get_irq probe defer case

Manage the -EPROBE_DEFER error case for the wake IRQ.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>