mfd: Convert AB3500 to new irq_ APIs

The genirq core is being updated to pass struct irq_data rather than irq
numbers into chip drivers. As part of the update assignments to NULL for
unused operations are removed, these are not needed and the genirq docs
should be good enough.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>