irqchip: vic: Properly chain the cascaded IRQs

We are flagging the parent IRQ as chained, then we must also
make sure to call the chained_irq_[enter|exit] functions for
things to work smoothly.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: http://lkml.kernel.org/r/1397550484-7119-1-git-send-email-linus.walleij@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>