gpio: omap: fix static checker warning

This patch fixes below static checker warning by changing
type of irq field in struct gpio_bank from u16 to int.

drivers/gpio/gpio-omap.c:1191 omap_gpio_probe()
	warn: assigning (-6) to unsigned variable 'bank->irq'

drivers/gpio/gpio-omap.c
  1188          bank->irq = platform_get_irq(pdev, 0);
  1189          if (bank->irq <= 0) {

bank->irq is u16.

  1190                  if (!bank->irq)
  1191                          bank->irq = -ENXIO;

Does not work.

  1192                  if (bank->irq != -EPROBE_DEFER)

Does not work.

  1193                          dev_err(dev,
  1194                                  "can't get irq resource ret=%d\n", bank->irq);
  1195                  return bank->irq;
  1196          }

Fixes: commit 89d18e3a: "gpio: omap: switch to use platform_get_irq"
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>