gpio: merrifield: Restore use of irq_base

During conversion to internal IRQ chip initialization the commit
  8f86a5b4 ("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: 8f86a5b4 ("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>