gpio: xgene: fix type of variable containing error codes

ret variable can contain error values and is compared with zero.
Its type must be signed.

The problem has been detected using coccinelle script
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-xgene-sb.c

@@ -240,7 +240,7 @@ static const struct irq_domain_ops xgene_gpio_sb_domain_ops = {
 static int xgene_gpio_sb_probe(struct platform_device *pdev)
 {
 	struct xgene_gpio_sb *priv;
-	u32 ret;
+	int ret;
 	struct resource *res;
 	void __iomem *regs;
 	struct irq_domain *parent_domain = NULL;