mfd: Fix asic3_gpio_to_irq

Assumption that irq numbers of asic3 gpios start at
IRQ_BOARD_START is certainly wrong - driver may as well
use any other base for its irqs (consider for example
the imaginary case of two ASIC3 chips onboard)

Furthermore, some platforms even don't have IRQ_BOARD_START
defined, so driver will fail to build on them:
-------------------------------------------------------
drivers/mfd/asic3.c: In function 'asic3_gpio_to_irq':
drivers/mfd/asic3.c:530: error: 'IRQ_BOARD_START' undeclared (first use in this function)
drivers/mfd/asic3.c:530: error: (Each undeclared identifier is reported only once
drivers/mfd/asic3.c:530: error: for each function it appears in.)
-------------------------------------------------------

Fix it by using irq_base from driver data.

Signed-off-by: Dmitry Artamonow <mad_soft@inbox.ru>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/asic3.c

@@ -527,7 +527,9 @@ static void asic3_gpio_set(struct gpio_chip *chip,
 
 static int asic3_gpio_to_irq(struct gpio_chip *chip, unsigned offset)
 {
-	return (offset < ASIC3_NUM_GPIOS) ? IRQ_BOARD_START + offset : -ENXIO;
+	struct asic3 *asic = container_of(chip, struct asic3, gpio);
+
+	return (offset < ASIC3_NUM_GPIOS) ? asic->irq_base + offset : -ENXIO;
 }
 
 static __init int asic3_gpio_probe(struct platform_device *pdev,