gpio: mvebu: make mvchip->irqbase signed for error handling

There is a bug in mvebu_gpio_probe() where we do:

	mvchip->irqbase = irq_alloc_descs(-1, 0, ngpios, -1);
	if (mvchip->irqbase < 0) {

The problem is that mvchip->irqbase is unsigned so the error handling
doesn't work.  I have changed it to be a regular int.

Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>