irqchip/stm32-exti: Prevent illegal read due to unbounded DT value

The value hwirq is received from DT. If it exceeds the maximum
valid value it causes the code to address unexisting irq chips
reading outside the array boundary.

Check the value of hwirq before using it.
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220606162757.415354-4-antonio.borneo@foss.st.com

drivers/irqchip/irq-stm32-exti.c

@@ -713,6 +713,9 @@ static int stm32_exti_h_domain_alloc(struct irq_domain *dm,
 	int bank;
 
 	hwirq = fwspec->param[0];
+	if (hwirq >= host_data->drv_data->bank_nr * IRQS_PER_BANK)
+		return -EINVAL;
+
 	bank  = hwirq / IRQS_PER_BANK;
 	chip_data = &host_data->chips_data[bank];