irqchip/bcm2712-mip: Fix OF node reference imbalance

irqchip/bcm2712-mip: Fix OF node reference imbalance

The init callback must not decrement the reference count of the provided
irqchip OF node.

This should not cause any trouble currently, but if the driver ever
starts probe deferring it could lead to warnings about reference
underflow and saturation.

Fixes: 32c6c054661a ("irqchip: Add Broadcom BCM2712 MSI-X interrupt controller")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>