irqchip/stm32-exti: Fix irq_set_affinity return value

When there is no parent, there is no specific action to do in
stm32-exti irqchip. In such case, it's incorrect returning an
error.

Let irq_set_affinity to return IRQ_SET_MASK_OK_DONE when there is
no parent.

Signed-off-by: Ludovic Barre <ludovic.barre@foss.st.com>
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220606162757.415354-2-antonio.borneo@foss.st.com