irqchip/stm32-exti: Fix irq_mask/irq_unmask for direct events

The driver has to mask/unmask the corresponding flag in the
Interrupt Mask Register (IMR).
This is already done for configurable event, while direct events
only forward the mask/unmask request to the parent.

Use the existing stm32_exti_h_mask()/stm32_exti_h_unmask() for
direct events too.

Signed-off-by: Loic Pallardy <loic.pallardy@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/20220606162757.415354-3-antonio.borneo@foss.st.com