mfd: arizona: Check if AOD interrupts are pending before dispatching

Previously the arizona_irq_thread implementation would call
handle_nested_irqs() to handle AOD interrupts without checking if any
were actually pending. The kernel will see these as spurious IRQs and
will eventually disable the IRQ.

This patch ensures we only launch the nested handler if there are AOD
interrupts pending in the codec.

Signed-off-by: Simon Trimmer <simont@opensource.wolfsonmicro.com>
Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>