irqchip/irq-mvebu-icu: Fix works by chance pointer assignment

Assigning a void pointer which points to a struct to two different data
types only works by chance if the second type is the first member of the
struct.

Replace this works by chance code by using the primary struct pointer.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221121140048.344525618@linutronix.de