mfd: Add to_irq() member to asic3 gpio_chip structure

The gpio_to_irq() macro is now defined as __gpio_to_irq() instead of IRQ_GPIO().
The __gpio_to_irq() function returns -ENXIO if the referenced gpio_chip
structure does not define a to_irq() member.
This is true of the asic3 gpio_chip structure, and thus calls to gpio_to_irq()
now fail (for example from the gpio-vbus module).
This patch defines the to_irq() member in the asic3 gpio_chip structure.

Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>