irqchip/stm32: Fix init error handling

If there are any errors in stm32_exti_host_init() then it leads to a
NULL dereference in the callers.  The function should clean up after
itself.

Fixes: f9fc1745 ("irqchip/stm32: Add host and driver data structures")
Reviewed-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>