gpio: merrifield: Restore use of irq_base

During conversion to internal IRQ chip initialization the commit
  8f86a5b ("gpio: merrifield: Pass irqchip when adding gpiochip")
lost the irq_base assignment.

drivers/gpio/gpio-merrifield.c: In function ‘mrfld_gpio_probe’:
drivers/gpio/gpio-merrifield.c:405:17: warning: variable ‘irq_base’ set but not used [-Wunused-but-set-variable]

Assign the girq->first to it.

Fixes: 8f86a5b ("gpio: merrifield: Pass irqchip when adding gpiochip")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-merrifield.c

@@ -455,6 +455,7 @@ static int mrfld_gpio_probe(struct pci_dev *pdev, const struct pci_device_id *id
 	if (!girq->parents)
 		return -ENOMEM;
 	girq->parents[0] = pdev->irq;
+	girq->first = irq_base;
 	girq->default_type = IRQ_TYPE_NONE;
 	girq->handler = handle_bad_irq;